Basic Opposed Piston Engine Diagram

In the past decade there has been renewed interest in opposed piston engine technology.  Is this a modern technological upgrade of a nearly abandoned engine configuration creating a “disruptive new engine” with leading fuel efficiency?  Or, a “pretend design”, a name Don Clausing coined to describe designs that are different but offer no net benefits in cost or performance over existing products?  Having literally “grown up” at Detroit Diesel in the 70’s and 80’s I have a fondness of 2-stroke diesels and as an innovator have interest in unique engine architectures.  I would personally like to believe a new future is possible for something other than a conventional 4 -stroke diesels.  Let’s explore.

The Players

Today Achates Power is unquestionably at the forefront in the news and effort to develop and commercialize the technology in a diesel engine and will be discussed in detail. However, others have played or are playing  in the field.  EcoMotors attempted the commercialization of its opposed piston opposed cylinder diesel concept, Superior Air Parts has purchased the “Gemini” opposed piston technology with intent to commercialize it as a small aircraft diesel engine and Pinnacle has attempted the commercialization of a unique 4 stroke gasoline version of the opposed piston concept.

EcoMotors ( as an entity appears to have been liquidated in late 2017 with the sale of their physical assets and intellectual property.  I am unaware of who may have purchased the assets nor whether Chinese companies that had involvement with EcoMotors have any intent to  move to commercialization.  Their concept had the unique ability to offer a construction of an engine with fully balanced modules connected in series by clutches.  Thus, allowing “stepped” variable displacement to provide improved low power fuel efficiency in addition to benefits inherently claimed for the opposed piston concept.  Despite major venture capital support, by people like Bill Gates, and claims of superiority over conventional diesels, the writer knows of no published test data confirming this nor the cost benefit comparison justifying the design.  One can only ask why the failure of the commercialization attempt; a failure to deliver results, poor execution of the concept, or financial issues limiting development?

The Superior Air Parts effort ( revolves around the commercialization of the “Gemini” design that has been under development in the UK for a number of years.  Several years ago, published rated BSFC curves for the original Gemini 100 showed minimum BSFC in the range of .400 -.410 lb/bhp-hr in the range of 1800-2500 rpm (  More recent publications show “potential” performance figures with BSFC values from .380 to .360 lb/bhp-hr for non turbocharged and turbocharged versions respectively without specific reference to operating point.  While these values are very respectable with respect to spark ignited aviation engines, they are poorer than many traditional 4-stroke diesels!  The primary commercial advantage of the product is not in world leading diesel efficiency.  It is in having a much higher level of efficiency than the gasoline engines, with which it will compete, while having competitive specific weight and package size and providing an aviation alternative to “leaded aviation fuels”.  It should be noted that the concept appears to use a simple/lower cost fuel system with a single injector per cylinder, similar to historically successful opposed piston engines as the Commer TS3, the Napier Deltic, and the Rolls Royce K60 engines.  Another good feature of the engine is the ability to take the propeller load off an intermediate gear as opposed to one of the two cranks.  This provides the ability to gear down, for a lower propeller speed, without the need of a separate gear box!

I recently attended the EAA airshow in Oshkosh, WI and visited the Superior booth. I was disappointed there was neither information available nor an engine on display.  When asked about the engine, it was indicated they would “have it out when it was ready” leading me to believe the task of commercializing it still has a long way to go.

While all of the previous engines were diesel 2-stroke opposed piston variants, the Pinnacle Engines approach ( differs in that it is a 4-stroke spark ignited gasoline concept operating in an opposed piston mode with a unique sleeve valve breathing arrangement.  The concept could also fundamentally be executed for a diesel cycle.  In being a 4 stroke cycle with cylinder gases entering and leaving at the top of the stroke(s), the fundamental problem of oil control across ports in the liners is solved  and scavenging challenges are eliminated.  The concept is not burdened by the 2-stroke wrist pin lubrication issues, the more severe piston thermal loading, nor the piston friction issues resulting from long and heavy. 2-stroke pistons.  Practically speaking, the entire piston cylinder technology is “carryover” from traditional 4 stroke engines.

The Pinnacle concept retains the fundamentally lower piston speeds and surface to volume ratios that provide the primary basis for improved thermal efficiency of the general opposed piston concept.  In addition it has the potential for good breathing from potentially large intake and exhaust areas.

On the downside however are oil control and lubrication issues of sleeve valve arrangement, well-known from similar approaches long since abandoned from World War II British aircraft engines.   Non ideal spark plug and or fuel injector positions and the challenge of achieving good upper bore cooling in an area where cooling competes with the breathing mechanization are also issues.  The moving mass of the valve system, it’s friction and dynamics are also potential issues.  Efficient packaging of multi-cylinder arrangement might also be of issue. Well engineered solutions are needed for the concept to succeed.

There are claims of “up to 50% better efficiency with little or no added cost”.   Despite such claims of efficiency gains, I am unaware of any published results showing demonstrated thermal efficiency improvement over benchmark conventional 4 stroke engines.  A comparison published shows a 33% mileage improvement over an undefined production engine in a motorcycle.  But, once again the base engine is undefined.  Forgive me for being a skeptic, but if the base engine was a older carbureted 2-stroke, the claim could be true but also unimpressive.

The primary initial market targeted by Pinnacle Engine appeared to be small single cylinder engines for the markets in India or China.  There are some indication that Greaves Cotton Ltd. in India has licensed the engine for production.  Let’s wait and see.

Publication of a couple of BSFC “hooks” at low, intermediate and rated speed in comparison to a “benchmark”conventional 4-stroke of similar power would go  long way on selling the concept.

At this point I will leave balance of the discussion to the 2-stroke opposed piston diesel concept.  All of the previously mentioned concepts share accepted principles that lead to higher thermal efficiency.

  • Reduced surface to volume ratio at near top center which should reduce heat losses at a critical time in the cycle
  • Reduced piston speeds for reduced friction

Obviously the extent of these advantages are associated with the relative BMEP (brake mean effective pressures) and the bore/stroke ratios employed.

Beyond these benefits are a gamut of factors affecting the competitiveness of the concepts relative to efficiency, specific output, specific weight, heat rejection, packaging, cost and exhaust emission attainment level.

Juxtaposed to potential benefits are the controversies associated with the 2 stroke opposed piston engine configuration:

  • Scavenging Issues
  • Basic Cycle Efficiency
  • Friction
  • Oil Control
  • Specific Output
  • Cost

When one considers a basic comparison of 4 and 2 stroke cycle engines, one often assumes a displacement of a 2-stroke engine at 50% that of a 4 stroke.  However, the blow-down event in a 2-stroke must occur earlier in the stroke and displacement is sacrificed for scavenging.  In practice the cylinder is never completely scavenged and the hot exhaust fraction heats the incoming charge resulting in lower charge density and less trapped oxygen. The net result is that for engines of comparable boosting technology and power,  the displacement of the 2 stroke generally needs to be approximately  67% of comparable the 4-stroke technology if not greater.  We can make a simple calculation for this by comparing  maximum  BMEP levels.  We find it common for 4 stroke diesels with single stage turbocharging and intercooling  to have rated BMEPs of 21 bar or higher.  The highest rating of medium and high-speed 2-strokes are about 14 bar for marine and military ratings using the highest rating of Detroit Diesel uniflow 2-strokes or the Wichmann loop scavenged engine.  Using the ratio of these BMEPs one can conclude the ratio is at best 67%.   Considering some 4 stroke diesels today are achieving maximum BMEP’s of 25 to 30 bar there is an implication that the displacement of the opposed piston engine may need to approach parity  (1:1) with highly rated 4-strokes unless maximum 2-stroke BMEPs can be raised.  Significant potential advantages in not only efficiency but also in size or mass can be lost if high BMEP levels cannot practically be obtained.

Achates Power (, as mentioned earlier, is unquestionably at the forefront of recent efforts to develop and industrialize applications of opposed piston diesel engines and has been active since 2004 in this effort.  Early patents show a significant effort to develop new crank arrangements, generally focused on package shape and or reduction of piston side loads.  Presently the development effort appear completely focused on using the conventional configuration with opposed crankshafts and connecting rods along with the recognition that the potential thermal efficiency benefit is primarily due to the improved surface to volume ratio and associated reduction of heat loss.  For all practical aspects the engine appears to be a rather conventional opposed piston engine configuration with modern updates in available technology such as high pressure common rail fuel injection, piston ring and liner surface design and aftertreatment adaptation.

High capitalization of the development effort and the pulling together of a highly skilled group of engineers to develop and market the concept speaks highly of the chances of success of the Achates organization.  I have personally known several of their engineers in my past work and can speak highly of them and their talent.  The organization has also published both modeled predictions of the performance of the concept as well as test results from prototype engines (SAE 2016-01-1019 et al.).  The publishing of test results is a big confidence builder relative to the earlier mentioned development efforts by others that published few if any technical results.

Achates’  recently supported work to upgrade the Fairbanks Morse 38 – 8 1/8 engine, now named “Trident” ( ).  My assumption is that the 38 – 8 1/8 engine had not had a significant level of upgrade in many decades and that  it would  certainly improve greatly with improved efficiency turbochargers and high pressure common rail fuel injection, which competitive 4 strokes are also adopting.  This work most certainly demonstrates how the older opposed piston technology can be made very competitive.  But, does it demonstrate a leading edge technology?

In the mentioned website, maximum efficiency of 50%* is claimed for the Trident – but following the “*” one finds the  additional footnote,  “Mechanical efficiency, optimized for fuel consumption”.  It is impossible to conclude from this if this level of efficiency can be maintained at Tier III or IV emission levels nor if this efficiency is at rated power or at some part load or lower speed rating.  I  have contacted Fairbanks Morse, but I have been unable to obtain any more detailed information on the fuel efficiency of the engine.  I  believe it is very safe to assume that very significant improvements of the engine’s thermal efficiency have been achieved.  However, for now I treat the 50% thermal efficiency claim with a reasonable level of both optimism and skepticism until there are actual applications of the engine and detailed performance data is published.

It is difficult to find an exact “apples to apples” type comparison of the Trident to a  new or recently upgraded 4 stroke in the same market.   If one accepts the new Wӓrtsilӓ 31 ( as a fair comparison, demonstrating latest 4-stroke technology, one finds an engine with a maximum  thermal efficiency of 51% with 48 to 50 % thermal efficiency  reported over a broad load range while at Tier III emission ratings.  The obvious conclusion is that the Trident may be approaching this level of performance but not exceeding it!  Similarly, comparing the claimed oil consumption of the Trident to that for the Wӓrtsilӓ 31 we find the Trident 70% higher than that claimed by for the Wӓrtsilӓ 31.   The Trident’s oil consumption would appear to be very good if not excellent for a 2-stroke and perhaps as good as some 4-strokes but far from leading edge.   Further discussion of oil consumption appears later in this article.

Achates’ marketing efforts often refer to the historical fuel economy results of the Junker 205 aircraft engine as evidence of the potential of the opposed piston engine.  There is no question that this engine demonstrated good efficiency for the 1930’s and 40’s.  But, is this a good argument for the concept?  The engine demonstrated good efficiency at lower speeds and lighter loads but was only at about 34% thermally efficiency at rated power. A quick search of aircraft diesels being developed in WWII shows a the KHD 710dz, a loop scavenged 2-stroke, demonstrating 40% thermal efficiency at rated power! My development experience would indicate single crank conventional 2-stroke diesels, whether loop or uniflow will only approach good 4 stroke diesels, with their primary advantage being in size or specific weight.

Achates’s argument from a historic argument, shown below, has flaws.

OP Best Point BSFC Comparison


It fails to illustrate there was little if any progress in improvement in efficiency of opposed piston engines that existed through the 70’s, 80’s and 90’s, the with best thermal efficiencies generally still being in the area of 37%-40%.   Several opposed piston engines existed in this period.  Examples:

  • Doxford
  • Kharkov 6TD
  • Fairbanks Morse 38D81/8
  • Rootes TS3 engine
  • Napier Deltic engine
  • Rolls Royce K60
  • Leyland L60

If the inherent benefits are there with opposed pistons, why did development progress stall?   Do the reasons still exist or are they displaced by modern engineering and manufacturing solutions and a need for reduced CO2?

A further error in this graphical argument is that the line shown for “Diesel Engines” does not represent the best 4-stroke diesel technologies.  There is no doubt there were commercial diesel engines with there best BSFC in the range shown, but there were better.  In the same era as the famous Junkers OP Engine was the 4-stroke Clerget aircraft diesel (   In 1939 it was demonstrating .375 lb.hp-hr bsfc under cruise conditions and one could  assume lower at “best point” part load and lower speed.  In the 1960’s Gardner 4 stroke diesels, known for excellent efficiency, were demonstrating 42% TE.  I can personally recall 4-stroke engines at Detroit Diesel in the late 1980s demonstrating best BSFC of about .300-.310 lb/hp-hr or below, or about 44 to 46% TE!  Multiple 4 stroke development efforts today are targeting and approach the 50% TE goal!  There are no questions that many diesel engines through history, opposed piston, 4-stroke and 2-stroke had best point BSFC worse than the figures stated.  The construction of many early diesels often focused more on achieving high power levels at low manufacturing costs and the ability to generate power with a relatively cheap fuel.

Modifications of the diagram, below, perhaps more realistically shows the trends of the diesels, I have not attempted an assessment of gasoline engines.

OP Best Point BSFC Comparison

There was literally no growth in the efficiency of commercial opposed pistons for decades after the historic Junkers engines until the more recent recognition and significant investment into the technology by Achates and others previously mentioned.  Growth in efficiency of 4 stroke diesels was higher for several reasons.   First, it is unquestionable that the development costs of 2-stroke diesels, whether opposed piston, uniflow or loop, are significantly higher than that of 4 stroke diesel engines discouraging  their commercial development.  Significantly more effort is needed relative to developing oil control, dealing with thermal loading, scavenging and combustion.  Secondly, the cost of producing such engines is generally higher due to the more complex structure of the components, gear train and air system.  As a result, commercial efforts did not focus on OP, despite its theoretical cycle efficiency superiority.

It should also be pointed out that fairly comparing the fuel efficiency of  various engines  in recent years has been more difficult because of the interaction and trade-offs involved in exhaust emission attainment relative to the use of varying degrees and use of intercooling, exhaust gas recirculation and SCR (selective catalytic reduction).  These all interact with the efficiency and cost of the application.  As highly efficient SCR is embraced in large engine applications, engines appear to be returning to operating at near peak thermal efficiency relative to combined fuel/urea costs and or the economics from reduced use of EGR .  In lighter duty applications there may be trade-offs in efficiency for reduced use of urea or total package costs.  Comparisons must be made very carefully!

From a historical point of view I have also looked at the Doxford OP large marine engines and compared their efficiencies to their 2-stroke competitors with uniflow (i.e. Burmeister and Wain) or loop scavenged engines (i.e. Sulzer).  While I am sure the Doxford engines provided value to their customers, it wold appear from a quick look through published data in older editions of Pounder’s Marine Diesel Engines , that the 2 stroke competitors to this OP execution had perhaps as much as a 10% fuel economy advantage relative to minimum BSFC.  It is not entirely clear to me why such a difference existed as all these engines shared high stroke to bore ratios.  It is also clear that the OP concept was abandoned by all major marine builder in later years over the uniflow design in pursuit of high-efficiency through long stroke design.

Achates’ tries to make an argument that opposed piston engines have faster combustion, a diagram as shown below has been used in several of their articles.

Heat Release Comparison from SAE 2014-01-1187
Heat Release Comparison of 4 Stroke and Opposed Piston Diesels (Ref SAE 2014-01-1187)

There is no question that faster combustion will improve thermal efficiency in a diesel engine.  What perhaps is not so obvious is that a 2-stroke diesel “must” have faster heat release due to it’s faster expansion rate relative to a 4 stroke engine with a comparable effective expansion ratio.  If not, efficiency will be sacrificed!  The formula for faster heat release is alluded to in one of my other articles,, in that increased injection rate and pressure in combination with proportional changes in air motion will speed up combustion and the rate of heat release.  Advancing timing such that more combustion takes place near TDC (top dead center) can also be effective.  These techniques work for both 2 and 4 stroke cycles and are constrained similarly by peak combustion pressure limits, NOx levels and or engine noise.  I have long felt it was the high pressure unit fuel injectors of General Motors, with generally more than twice the pressures of many competitive 4-stroke diesels, that allowed these 2-stroke diesels to have the fast combustion needed to be successful in their heyday.

There is no doubt that the 2 independently controlled nozzles per cylinder in the Achates approach may offer a benefit over single nozzle common rail system to achieve the faster needed heat release.  It can also minimize difficulties at light load by reverting to single nozzle operation.  However, the down side to a two nozzle per cylinder approach is twice the nozzle sac volume exposure rate and the corresponding negative effect on HC (hydrocarbon) emissions.  A three cylinder, 2 stroke engine with a single injector has the same sac exposure rate as a 6 cylinder 4 stroke engine and thus similar sac volume related HC emissions.  A two nozzle per cylinder system will double the injector sac HC emissions.  Minimizing the sac volume in proportion to the injector size is generally not practical and VCO type nozzles, that can nearly eliminate sac related HC emissions, are falling out of supplier product offerings for structural reasons as maximum injection pressures are rising from 2000 to 2500 and even 3000 bar.

Higher HC’s do not pose a significant problem with use of exhaust oxidation catalysts once operating temperature is reached and maintained.  However, for emissions standards such as SULEV-30, the NOx +HC’s in the emissions cycle engines can easily exceed the limits in the warm-up period alone, when aftertreatment is not active.   In the warm-up phase, HC’s levels have importance.   HC’s related to long ignition delays or surface quenching can be improved significantly in 2-stroke diesels by increasing in cylinder temperatures by limiting the engines scavenging airflow – but this will not affect sac related HC’s.

A second issue exists with single nozzle operation in a two nozzle per cylinder system.  Nozzle overheating problems are probable for the non-firing injector that does not have fuel flowing through it unless special cooling considerations are made.  Use of higher alloy nozzles and or nitrided nozzles might also be an option.  Alternating firings of the injectors may be a means of preventing overheating of the nozzles by keeping some fuel flow through both nozzles.

An interesting sidebar on the number of nozzles used per cylinder is that the Junkers Jumo 205E engine, highlighted for its demonstration of high thermal efficiency, utilized two unit pumps per cylinder with 4 injector nozzles arranged symmetrically around the combustion chamber.  This is in contrast to a single pumping element per cylinder and a single injector with a single hole nozzle used in several of the commercially produced OP engines discussed earlier.  Generally symmetry in diesel combustion systems has played a role in the best combustion systems with even small offsets in bowl or injector location providing incremental losses in efficiency and or emissions.  Large uniflow ultra long stroke 2-stroke engines almost exclusively use three nozzles injecting with the swirl from the periphery of the chamber. Could the use of four nozzles have been a key to the Jumo 205E’s success?

Another significant challenge for Achates is oil consumption and upper cylinder lubrication.  When one looks historically at Detroit Diesel two strokes one finds a history of the need for “looser” oil control as BMEP’s were increased with increased injector size and boosting.  Detroit Diesel Series 71 2-strokes traditionally used 3 different oil ring combinations.  The tightest control was for not turbocharged engines with smaller injector sizes (below 60 mm3 per stroke), a second level of oil control was for non-turbo engines with higher output injectors (60 mm3 and above) and a third level of oil control was used for all turbocharged engines with the higher power ratings.  The use of these three levels is related to cylinder life and reliability.  The challenge opposed piston engines have is to have one ring package that will maintain low oil consumption at all operating conditions while providing good upper cylinder lubrication for high load operation.

Wickmann WX28 2-stroke diesels used a 3-ring groove oil ring arrangement for oil control and claimed “virtually no use of crankcase oil”.   However, their engines also used a quill system of lubrication to meter oil directly onto the upper cylinder wall similar to large cross-head uniflow 2-strokes, where this is done by necessity.  Metering oil directly onto the wall allows oil consumption to be programmable with load and speed to meet the cylinders need.  The rated power metered oil consumption of these engines, 1.4 g/kw-hr,  is at the generally higher level associated with 2-strokes, this is justified or associated with the prevention of cylinder and ring problems associated with the use of heavy fuels in this marine engine.  I would assume, this level be lowered with ultra low sulfur diesel  fuel.  Could such a system be simplified and reduced in cost for use in smaller opposed piston and 2-stroke diesels?  I suggested such and approach in US Patent 9,004,039 B2.  If such a system could be worked out in a practical and cost-effective manner, it could provide a means of assuring both high load cylinder reliability and low part load oil consumption.

The mechanism of oil transfer from below the intake ports (or intake and exhaust ports in opposed piston engines) to the critical top ring reversal area, is quite complex and much more challenging than in 4-stroke engines.  In 4-stroke engines much of the cylinder can be heavily lubricated with the oil and nearly totally “scrapped down” and replaced with fresh oil on each stroke.  In ported 2-stroke engines this is only possible below the ports and the oil transfer across the ports must be from the ring overlap area where both the compression and oil rings will cross the same area in the bore  and or from transfer from the cylinder bore below the ports to the piston skirt and then from the piston skirt back to the bore above the ports.  All transfer must occur with a nearly no oil being lost while the rings and skirt pass the ports.  Some oil scrapped off at the port may be blown through the cylinder during scavenging and may contribute to bore lubrication, albeit much is blown through into the exhaust where it results in a heavy “wet” particulate emission.

The good part about the 2-stroke type of “minimal oil transfer” is that the oil in the crankcase stays remarkably clean.  This, in combination with a lack of blow-by gases directly into the crankcase, provides opportunity for much longer oil change periods.  At one point in Detroit Diesel history “100,000 mile” oil change intervals were allowed in 2-stroke highway trucks if oil quality was monitored!  EMD (Electro-Motive Diesel) offered better total “oil economy” with their 2-stroke diesels despite significantly higher oil consumption than their 4-stroke GE (General Electric) locomotive competitor due to the reduced oil change level needed.  If “overall”  oil economy is an issue, 2-stroke diesels can do well.  But actual oil consumption brings other issues.

The negative issue with the 2-stroke “minimal” oil transfer is that oil that reaches the upper cylinder must stay there until depleted by combustion and or evaporation from power strokes exposing it to high temperature gases, radiation, soot and or un-burned fuel every stroke of the piston.  This can leave the oil on the wall in a more degraded state, particularly if oil consumption is very low.

It is always interesting to note that throughout history, Detroit Diesel recommended only use of single weight oils in all of their 2-stroke applications, even in non-turbocharged engines with BMEPs less than 7 bar.   While in earlier times this was 30 weight oil, in more recent times 40 weight oil is required and in some cases 50 weight oil is recommended.  A “shear down” mechanism where a multi-viscosity would revert to the level of protection of the base molecule under severe conditions of temperature and stress can explain why a high weight oil historically would be preferable in a 2-stroke engine.  This recommendation was continued throughout the production period of these engines through the late 1990’s and continues to appear in their Fuels and Lubricants Specifications today .

Detroit Diesel - MTU 2-Cycle Oil Recommendation

From page 93 Fuels and Lubricants Specifications (DD/MTU document)

SAE 15W-40 oils were only allowed in the 2-strokes under cold ambient conditions and were never allowed in the higher BMEP rated marine engines.

Detroit Diesel Oil Recommendations
From :  Detroit Diesel Document “Lubricating Oil, Fuel, and Filters”

Perhaps advancements in oil viscosity improvers in recent decades has increased the High Shear Rate Viscosity sufficiently to allow the use of multi-viscosity oils in 2-stroke opposed piston diesels.  However, I’m not aware of  any 2-stroke durability  tests that could substantiate this.

While recently reviewing literature on Mobil 1™ Turbo Diesel 0W-40 motor oil (  I found they claimed that “Mobil 1 Turbo Diesel 0W-40 is especially suitable for extreme conditions, where conventional oil often may not perform”.  However, they also limited the claim by noting “Mobil 1 Turbo Diesel 0W-40 is not recommended for 2-Cycle or aviation engines, unless specifically approved by the manufacturer”.  I believe this speaks to the concern about the severity of the 2-stroke diesel lubrication environment.

It is interesting that EMD did eventually allow the use of specially formulated 20w-40 oils for their “low emissions” 2-stroke locomotive diesels operating with Low Sulfur and Ultra Low Sulfur Diesel Fuels.  EMD previously, like Detroit Diesel, had a “single weight only” position (   This may be more evidence that a specially formulated oil may be needed for the opposed piston engine in the future.  The EMD 710 engine has a maximum BMEP of about 11.4 bar at 950 rpm.  One can question whether this oil would provide adequate protection at higher BMEPs i.e. 14 bar or higher.

There is no doubt that Achates work to develop good upper cylinder cooling is well justified.  Use of “Bore cooling” in larger diesel engines has long been a practice in large diesel engines of both 2 and 4 stroke types and many modern smaller 4-stroke diesels focus on this.  With 2-stroke diesels it is of particular significance due to the short time between power strokes for cooling of the critical ring reversal area to preserve good lubrication.

The need for low ash oil for Detroit Diesel 2-strokes was always a necessity to avoid both deposits and ring sticking due to the higher piston temperatures and the higher lever of oil consumption.  Today such oils for are widely available and there use mandatory with advance aftertreatment to avoid premature plugging of catalyst and particulate filter substrates.

Achates has indicated they have demonstrated oil consumption at “4-stroke levels”.  As with the earlier comparison of Trident and Wӓrtsilӓ 31, I have my personal concerns that this may not be as compared to the “best” 4 strokes, nor that it may come without the application of a higher level of technology to achieve cylinder life.  Fortunately, a wide range of cylinder finishing, texturing and spray-bore coatings  are available to support this (ref ASME Paper ICEF2012-92083) as well piston new oil ring technology.  Applying different honed finishes to different parts of the liner and or use of laser honing or texturing to provide adequate oil retention are all possible.  They will likely need to be applied to a greater extent and level of control for the 2-stroke diesel.  Perhaps differing levels of control at the the exhaust and and intake sides of the cylinder will be needed.  A great example of this type of finishing can be found in EMD (Electro-Motive Diesel) cylinder liners and is described in great detail in their US patents 7,162,798 and US9387567B2.

Other Opposed Piston Engine Considerations

Relative to the liner bores in 2 stroke engines, several other things come to mind. If one studies the history of EMD 2-stroke development.  It was found quite early these engines suffered from piston scuffing emanating from the liner port area of the liner.  What was discovered was that with the combination of forced bridge cooling of the liner port bridges and the cooler air going through the ports caused the liner to expand less at the port belt reducing the bore locally causing scuffing.  The solution was to finish the port belt to a larger diameter.  To the best of my knowledge, this feature has been retained to present times.

It is interesting to note, that I cannot recall any such intentional bore shaping of the port belt in traditional Detroit Diesel 2-stroke liners.  A minor amount of diameter enlargement might naturally be caused by the honing stones cutting more aggressively in the port belt region.  However, what I can recall was that in late development of highly intercooled Series 92 2-stroke engines (ref SAE 850317), scuffing was occurring at the liner ports.  The solution was a special liner with “Brushed Ports” that was used in some applications, part number 23507231.

Two other development issues are related to the intake ports on 2-stroke engines.  One is port plugging.  This is a phenomena where blow-back through the intake ports late in the blown-down process interacts with lube oil scraped off at the ports and or soot in the gases to cause deposit build-up in the liner ports.  I recall all of the Detroit Diesel engines could be susceptible to this relative to operating conditions.  In non-turbo 71 Series engines it was avoided by advancing exhaust cam timing by one tooth (4.6 degrees) to start the blow-down process earlier, but at a slight loss in efficiency due to the shortened expansion stroke.  All turbocharged engines designed in sufficient blowdown time area to avoid the issue in the majority of applications.  The deposits were often soft and oily and would both form and break away in normal operation, but could restrict airflow and or distort the air entry into the cylinder affecting swirl level and combustion.  I can also recall that there was a tendency to form extremely hard, almost “ceramic like” deposits in the ports when there where efforts to reduce oil consumption!  Obviously opposed piston engine developers must address these issues to avoided problems throughout the life of the engine.  The issue of oil loss at the exhaust ports creates a new situation for deposits for which I have no direct experience.  The Detroit Diesel Series 51 engine, which was looped scavenged had exhaust ports in the liner.  Plugging of both intake and exhaust ports are listed in the Trouble Shooting section of the operators manual and inspection and or cleaning of the ports every 1000 hours was recommended.

The other phenomena related to the intake ports of two stroke engines took the unfortunate name of “slobber” at Detroit Diesel.  Slobber was the term for the accumulation of oil and fuel in the airbox of the engine and or of this mixture being blown through the engine into the exhaust.  In cold operation of a diesel, fuel condenses on cylinder walls.  In 4-stroke engines this mechanism results in oil dilution as the mixture is scrapped off the cylinder by the oil rings.  In 2-stroke diesels, this fuel/oil mixture is scrapped into the liner ports where it can accumulates in the air plenum (and exhaust plenum in the case of OP engines) or is blown through the engine into the exhaust.   In Detroit Diesel 2-strokes, strategically placed airbox drains, drained the mixture accumulating in the air plenum either into a collection container, onto the ground, or into the bilge of a boat.  In later days environmental an aesthetic concerns caused the drains to be routed back to the crankcase often with check valves that would close at airbox pressures above that of idle in order to avoid air loss or cause excessive crankcase pressure and breather flow.  In cases where significant amount of the oil/fuel mixture accumulated in the airbox due to plugged drains, the mixture could be sucked into the engine during acceleration.  This condition could cause a “run away” engine due to the uncontrolled fueling and would often result in overheated injector nozzles from high temperatures generated by very early ignition and subsequent compression.

Oil/fuel mixture that accumulated in the exhaust would result in white smoke and or a dripping mess from the exhaust.  This brings to mind a story told to me by my first supervisor, Dick Hames.  As I remember his story, as young engineer he went to an airport or military base to address a white smoke complaint emanating from a Series 53 2-stroke diesel generator set that had been running at very light loads.  Having found a “very slobbered up” horizontally oriented exhaust system, he decided he needed to get some load on the engine to clear it up.  He added a load bank to the generator and took the engine to a high load.  Upon doing this the mixture in the exhaust unintentionally was ignited resulting in huge flame shooting out the exhaust pipe towards the hangar, nearly setting it on fire!

I don’t want to leave an impression that this was a systemic problems in Detroit Diesel engines, and for most any such issues were minor and accepted inconveniences.  However, with the right combinations of temperature, injector timing, low compression ratio and lower cetane the issue could become a major nuisance.

Some degree of the “slobber” phenomena under cold start operation at light load is virtually unavoidable in 2-stroke diesels and must be dealt with.  With little or no field experience it is doubtful that any of the recent opposed piston developers has fully experienced or addressed the issue and or it’s potential effect on aftertreatment systems. The use of late post injections for aftertreatment warm-up and or regeneration would be anticipated to result in the mechanism.  This same mechanism exists in 4-stroke diesels where it manifests itself only in oil dilution.  It is likely the opposed piston engine may need to focus on direct injection of fuel into the exhaust stream for particulate filter regeneration versus use of late in-cylinder injections.  Good thermal management of the cooling system, use of the highest practical compression ratio’s, hot internal EGR and advanced injection timings at light load would appear to be the obvious approach to minimizing the potential for the issue under light load cold operation.

Piston design provides a challenge for opposed piston engines, particularly those with longer strokes as the piston length must exceed the stroke to keep the oil rings below intake and exhaust liner ports.  The high skirt contact area, piston mass and number of rings works against any inherent friction advantages of the opposed piston engine. Recent trends in 4-stroke diesels are towards steel versus the traditional aluminum pistons and towards greatly reduced skirt areas to lower both the mass and the friction of the skirt.  Reduced thermal expansion with steel pistons also provides more optimal clearance for friction reduction across the operating range.  Higher temperature of the steel dome is felt by many to provide a path the improved thermal efficiency.  This feature has been used almost exclusively in all historic 2-stroke and opposed piston engines and is thus not available as a path to them for major improvement.  The high thermal loading of the exhaust piston rim and top ring in the opposed piston engine makes the piston design more challenging than that of traditional uniflow two stroke diesels.

Evolving thermal barrier coatings should prove beneficial to both 2 and 4 stroke diesels.  But, with the application to both the piston and head in 4-stroke engines the heat loss  benefit of the opposed piston concept will be reduced relative to the conventional 4-stroke.

The cost of opposed piston engines relative to a 4-stroke diesel can be debated. The most direct comparison is probably the 3 cylinder opposed piston engine to the traditional inline 6 diesel.  Both engines have similar torque smoothness with the same number of power strokes per revolution and both engines have good inherent balance.  I believe the cost of the opposed piston engine will be higher. I will not try to do a detailed comparison of costs with so many details dependent on actual designs.  I will offer the following points for consideration.

  • Although both engines have the same number of pistons, one would expect the opposed piston engine to have higher cost pistons due to lack of existing suppliers, length of the pistons, number of rings and complex wrist pin design.
  • Although the opposed piston engine has no cylinder head, it has a very complex block structure and coring to construct it.  Bore spacing must be larger to provide for porting at the sides of bore resulting in more mass and material than one might expect.
  • Opposed piston cylinder liners are very complex and need to be anchored and oriented both axially and radially in the block bores to provide alignment with the fuel injectors. Liner ports must be machined and passages provided for bridge cooling.
  • Most automotive engines below class 7 or and 8 use engine blocks without liners so use of liners in opposed piston engines will add costs. A liner-less opposed piston  engine block with cast bores and all passages cast in through use of a printed block molds or by printed construction or additive manufacturing might be a future option.  I would assume such manufacture would only be appropriate for low volume niche market engines and probably at a premium cost.  However, this technology is evolving rapidly!
  • While valve train costs are eliminated in opposed piston engines, I believe the cost of the additional roots blower system and drive and control are approximately offsetting.
  • If E-boosting (electrically assisted turbochargers) becomes a market direction for 4-stroke competition, the opposed piston engine (and conventional 2-stroke engines) could minimize cost through use of a single electrically assisted turbocharger in place of the typical turbocharger-roots blower system resulting in cost savings.
  • The opposed piston gear-train must be larger in size and strength than the traditional valve gear and accessory one used in typical 4-stroke engines causing negatives in both friction and mass.
  • If 2 injectors per cylinder are used on opposite sides of the engine, fuel system costs will be higher due to the need for multiple fuel rails and longer fuel lines. The need for a short injector may limit choices and require more of a custom injector design. A single injector per cylinder fuel system could reduce costs due to the need for fewer injectors and associate parts but may affect efficiency.
  • Higher thermal loads on injector nozzles, in the 2-stroke opposed piston engine, may drive the need for premium nozzle material and or special nozzle cooling considerations.

Another consideration that must yet be addressed is cold starting.  For heavy duty engines, that traditionally have not used glow plugs, this is not an issue.  However, for smaller engines and the automotive market this will not be acceptable.  Can a glow plug position be engineered into an opposed piston engine?  Or, does a high wattage electric air heating system in the air plenum provide a better solution?

If exhaust gas recirculation (EGR) is needed in the 2-stroke opposed piston engine, hot internal EGR can be readily accomplished at part load by reduced scavenging accomplished by slowing down or bypassing the scavenging pump.  In many instances this actually reduces pumping work and reduces fuel consumption. This can be very valuable during under warm-up and cold light load operation. However, hot EGR is not highly effective for NOx reduction during normal operation due to its effect on charge temperature.

If use of cooled external EGR is needed in the opposed piston engine, a basic dilemma exists that 2-stroke diesels must have higher air supply pressure than exhaust back pressure on the cylinder to provide the scavenging airflow through the cylinder. As a result creative measures must be employed to move the exhaust gas through a cooler and into the air inlet stream. There are a variety of ways that this can be accomplished, the most straight forward method is to employ “long route” EGR where the exhaust gases are taken from after the turbine, particulate filter and aftertreatment directly into the turbocharger compressor, or to a cooler and then through the compressor.  Other methods might be considered.  For example, removing exhaust gas at higher pressure from before the turbine inlet and channeling it through a cooler and introducing it after the turbo compressor but prior to a final stage of compression by the roots blower.  However, with this second method comes high probability of cooler fouling due to the previously mentioned oily wet soot and or deposit problems in the scavenge pump normally a roots blower.  Another option that is evolving for 4-stroke engines is an EGR Pump – this obviously could also be applied to the 2-stroke opposed piston engine.


The opposed piston engine unquestionably has a potential thermal efficiency advantage over conventional 4 stroke configurations due to surface to volume advantages and low piston speeds it offers.   However, for the product to be successful in a broad market, there must be engineered solutions to the multitude of issues mentioned in this article as well as solutions for unknowns that will be realized in the early application.

The conventional 4-stroke diesel continues to improve through competition between a multitude of engine makers, suppliers, and technologies.  This provides a moving target for the opposed piston engine.  Some of the technologies evolving can apply to opposed piston, others many not.  Competing technologies and companies drives down cost – a major challenge for new parts and manufacturing techniques for a new engine concept.

What is the future of opposed piston technology?   “Pretend Design” or “Disruptive New Product”?  Time will provide the answer.





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