OPGCI: An Evolution that Revolutionizes the Internal Combustion Engine

By Fabien Redon and Steve Ciatti

Automakers are working hard to meet pending fuel economy and emissions standards, like CAFE 2025 (54.5 mpg), and investing heavily in new technologies such as electric vehicles and their variants. According to a Frost & Sullivan forecast, however, 105,000,000 passenger and light commercial vehicles will be sold in 2020 – 98.6% of them with internal combustion engines.

Recently the Secretary of Energy, Dr. Ernest Moniz, announced an award to Achates Power, Argonne National Laboratory and Delphi Automotive, to develop a gasoline compression ignition (GCI) version of the Achates Power opposed-piston engine. The grant is one of the largest awarded by the Advanced Research Projects Agency – Energy (ARPA-E) in its history.

An opposed-piston, gasoline compression ignition (OPGCI) engine has the potential to be a game changer in the powertrain market, with very clean and efficient power. The combination of the two technologies could be the solution to pending emissions and fuel economy regulations and could very well be the internal combustion engine (ICE) that satisfies the challenges of ground mobility for decades to come.

The OPGCI combines proven, efficient technologies in an engine that has the potential to be about 50% more efficient than today’s gasoline engines, with comparable power; torque; noise, vibration and harshness (NVH); and, size. It does this by using the benefits of compression ignition, with a readily available fuel source – gasoline – in the highly efficient opposed-piston architecture, refined by Achates Power.

Compression Ignition

Diesel engines operate using compression ignition, and are about 40% more efficient than gasoline engines that operate on spark ignition.

The efficiency advantage of diesel comes from the 13% higher energy density of diesel fuel – the other 27% comes from efficiency advantages of compression ignition over spark ignition. Russ Durrett of General Motors summarized these advantages in a presentation at the University of Wisconsin Engine Research Center.

Lean combustion – excess air relative to fuel – provides the majority of that improved thermodynamic efficiency in compression ignition by diluting the fuel to air mixture, resulting in lower combustion temperatures and reduced heat loss.   For those mathematically inclined, we can refer to the equation for ideal engine efficiency:Ideal Engine Efficiency


Increasing air relative to fuel increases the ratio of specific heats (gamma, in the equation above). The reason for this comes from the ability of the gas to store the thermal energy created by combustion as increased pressure – i.e., how much of the temperature increase from combustion (what we pay for) goes into the increased pressure (what we get – work)?

Monatomic molecules, like noble gases (He, Ne, Ar, etc.) have the maximum gamma because they have almost no ability to store thermal energy aside from increasing the pressure of the gas. Unfortunately, noble gases are not reactive, so their use is challenging in a combustion system. Diatomic molecules, like nitrogen and oxygen, have a few ways to absorb thermal energy in their respective molecules but most of the energy goes into increased pressure. Poly-atomic molecules (CO2, H2O, etc.) have many more ways to store thermal energy without increasing the pressure; this type of energy storage is unrecoverable. As a result, to achieve the highest practical efficiency, the highest possible gamma is desired – which for practical purposes means that it is very desirable to use air as the primary working fluid of expansion and not exhaust gas. Operating the engine “lean” means that more of the working fluid is air and not exhaust gas. Spark-ignited gasoline engines, by contrast, operate at or near stoichiometry – the precise ratio of air to fuel to completely burn all the fuel – which creates the maximum amount of CO2 and H2O concentrations in the working expansion fluid.

Internal combustion engines are more efficient at high compression ratios. Another significant source of efficiency improvement (roughly one-quarter) comes from the increased compression ratio in diesel engines. If an engine has more compression it also (typically) has more expansion and delivers more power with the same amount of fuel input.  As expansion continues, the temperature of the exhaust gas is reduced as the energy is put into useful work.  Less energy is wasted as heat in high compression ratio engines.  Gasoline spark ignition engines are limited to lower compression ratios to control pre-ignition (knocking) which can damage the engine.

The last factor in favor of diesel engines – representing the remaining quarter of the efficiency gain – is reduced throttling losses.  Gasoline engines partially close a valve (the throttle) at low loads to maintain stoichiometry; if the fuel is reduced for lower power output, proportionally the air must be reduced as well to keep the fuel/air ratio constant. Hence a restriction is put into place to curtail the airflow, i.e., throttle. However, this restriction creates extra work required to move air into the cylinder and subtracts from efficiency. Think of a syringe with a very small opening, trying to suction a very viscous fluid, fast! It requires considerable effort to fill the syringe because of the restriction. When a throttle is mostly closed, this is the type of restriction that the engine fights against to get air into the cylinder.

Gasoline Compression Ignition

Delphi and Argonne have both demonstrated that gasoline can be combusted with a high compression ratio, under lean conditions and without throttling, and can be ignited without a spark plug.  The key is to provide the right pressure, temperature and fuel distribution inside the cylinder.  The development of gasoline direct injection systems has enabled GCI – gasoline can be injected into the cylinder at the right time to create a generally lean mixture, complemented with a final injection just before combustion to create the right air-fuel ratio locally for ignition.   Delphi has shown that their GCI engine offers diesel-like efficiency. Furthermore, GCI has an advantage over diesel in creating lower emissions.

NOx is formed at high temperatures. Even though diesel is operating lean, on average the combustion is mostly happening in local richer mixtures leading to high local temperature causing NOx and soot formation. 

Gasoline is a superior fuel for compression ignition because gasoline evaporates more readily than diesel and has a longer ignition delay. GCI has a mostly lean mixture more evenly distributed throughout the cylinder with only a small portion of richer mixture at the ignition sites it therefore achieves mostly lower peak temperatures and NOx.  In addition, the mostly lean local conditions also allow for low soot formation.  GCI does, however, create higher hydrocarbon (HC) and carbon monoxide (CO) emissions.  Fortunately, HC and CO can be mitigated with relatively inexpensive oxidation catalysts.

Another advantage GCI has over diesel is lower cost, both because of much lower cost aftertreatment requirements (GCI engines generally do not need a particulate filter and may not need selective catalyst reduction) and because of much lower cost fuel system.   Diesel, as a heavy fuel, requires high injection pressure to adequately atomize.

Delphi recently published results of experiments that yield 39.3% MPG improvement in combined city and highway drive cycles for a GCI engine compared to a 2.4L four cylinder port fuel injected (PFI) engine.

Achates Power OP EngineThe opposed-piston engine

Achates Power has spent 12 years improving upon the technology of the opposed-piston engine. The design of the architecture also addresses many of the challenges that GCI has faced.

The OP Engine eliminates the cylinder heads so it has an improved surface area – to – volume ratio of the combustion chamber (i.e. less surface area relative to the volume) for reduced heat transfer and heat rejection. This has numerous benefits:  Less heat is wasted through the cooling system, enabling more of the fuel energy to be used for useful work; reduced heat transfer enables earlier and more efficient combustion; and reduced heat rejection to coolant enables reduced cooling system and radiator size resulting in reduced aerodynamic drag of the vehicle.

The OP Engine takes advantage of the inherent power density of a two-cycle engine by reducing both displacement (reducing the size, mass, and cost of the engine) and brake mean effective pressure (BMEP) at the same time.  Reduced BMEP results in low NOx operation and enables more rapid combustion to improve efficiency.  The reduced BMEP also results in leaner and more efficient combustion at the same boost level, which has the additional benefit of generating less soot.

The OP Engine has efficient, uniflow scavenging that decouples the pumping work from the engine speed.  At low loads, the engine can retain a high proportion of exhaust gases by reducing the supercharger work, improving efficiency while providing a natural exhaust gas recirculation (EGR) effect for low NOx combustion and high exhaust gas temperatures for catalyst light-off.

The combustion system developed by Achates Power uses diametrically opposed dual injectors with proprietary piston shapes to provide excellent fuel and air mixing, resulting in low soot combustion and reduced heat transfer to the combustion chamber walls.

Achates Power recently published results of experiments that yield 30% MPG improvement in combined city and highway drive cycles over a conventional diesel engine, while meeting U.S. EPA Tier 3 automotive emissions standards.

Combining OP & GCI

We expect that combined the OP Engine and GCI will result in a number of advantages.  Since the efficiency advantages of GCI and OP compound, the resulting engine could improve engine efficiency by about 50% over PFI gasoline engines. Likewise, since both technologies have favorable cost positions compared to conventional engines, the combined engine will as well.

The OP Engine also optimizes three technical challenges of GCI.

Mixture preparationGCI Injection

Robust and clean GCI combustion requires a stratified charge, with locally lean and rich areas, and multiple injection events.  Combustion initiates in locally rich areas.  Stratification then enables controlled heat release that prevents high NOx-forming combustion temperatures.

Delphi has achieved excellent GCI combustion results in conventional engine configurations, with an injector inserted through the cylinder head injecting towards an approaching piston (Figure 1).

The OP injection environment offers significant advantages for charge stratification.  Diametrically opposed dual injectors spray across the diameter of cylinder (Figure 2).  Each injector can be independently controlled to more easily manage staggered injections for just the right mixture distribution and, therefore, efficient and controlled heat release.

OP Injection Image

Charge temperature management

At low loads, GCI requires higher temperatures for combustion.  Engines operating at low loads (at idle or when coasting) generate relatively little heat. This problem is exacerbated in small engines that have high ratios of surface areas to combustion volume.  Four-stroke engines normally push the entire content of the cylinder out during the exhaust stroke and therefore require a complex variable valve train to re-open the exhaust valve during the intake stroke to suck the exhaust back in the cylinder to increase the charge temperature to the level necessary for GCI ignition.

The OP Engine, by contrast, can retain exhaust gas in-cylinder after combustion, especially at low loads when relatively little new oxygen is required. The amount of scavenging an OP Engine performs is determined by the pressure ratio between the intake manifold and exhaust manifold.   At low loads, the OP Engine can reduce the supercharger work used to boost the intake manifold pressure.  This has four benefits: it reduces the amount of work by the supercharger, improving efficiency; it keeps in-cylinder temperatures high for good combustion stability; it provides a natural or internal EGR effect for low NOx combustion; and, it provides high exhaust gas temperatures for catalyst light-off and sustained activity.

High Load Operation and Pumping

At the other extreme, GCI engines also have challenges at high loads.  The compression ratio of a GCI engine is higher than a conventional gasoline engine and also requires a higher level of air and EGR to control combustion.  This combination creates high cylinder pressures that can limit the maximum load capability of the engine. Without the high charge dilution at high load, the GCI combustion would be too rapid and create high combustion noise.  The high air and EGR levels required at high loads can add significant pumping work, which can hurt fuel consumption.  At high loads four-stroke GCI engines have to make calibration tradeoffs to maintain the mechanical integrity of the engine, sacrificing both efficiency and performance.

The OP Engine, by contrast, has several advantages to manage the high load operation without as many trade-offs:  The two-stroke cycle operation reduces the maximum BMEP requirement (and displacement) while maintaining performance requirements.  Relatively large flow area of the ports, better alignment to turbocharger performance curves and efficient EGR pumping all contributed to reduced pumping work to meet the necessary charge conditions.  Finally, the larger cylinder volume available for combustion enables faster heat release rates without increasing combustion noise.  This more favorable condition allows for fewer calibration tradeoffs at high loads and has the potential to achieve higher power levels and even better fuel consumption.

The OPGCI Engine can be the most cost effective and financially viable way to reduce greenhouse gas emissions because it leverages an existing infrastructure and manufacturing processes. The combination of the OP Engine approach (low heat transfer, high power from modest torque, excellent flow characteristics) with GCI combustion technology (lean operation, high compression ratio, no throttle, ultra-low emissions signature) offers the opportunity to evidence substantial gains in both efficiency and environmental friendliness. The OPGCI is well-poised to take advantage of exciting low-CO2 refinery streams such as naphtha and even newer biofuel processes that currently struggle with mimicking traditional gasoline or diesel fuel streams. The future is exciting indeed!

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8 thoughts on “OPGCI: An Evolution that Revolutionizes the Internal Combustion Engine

  1. Most downsized gasoline 4 cycle engines used in passenger vehicle are operating the majority of the time at part load conditions, and in city traffic even more so. With fixed compression ratios and part load running means that the thermal efficiency is at its worst.
    Dr Bill Mansfield back in the 1950’s invented a VCR piston for a BMW 1.5Lt L6 – 4 cycle gasoline engine. The CR varied from 6.5 : 1 and 16.5 : 1, on 70 octane fuel. The part load fuel economy was improved by 25%. He also invented a VCR piston for 2 cycle engines.
    I would be interested to learn if you are considering VCR to take OPE 2 cycle engines beyond the 50% BTE.

  2. Dear Martin, you are correct low speed low load operation, where most driver spend all their time is a very inefficient region of the operating range of an engine. It is clear that varying the compression ratio can allow for an engine that has less compromises to achieve high efficiency at low loads and can achieve high loads and stay within mechanical limits.
    There are many ways to change the compression ratio of an OP engine including changing the piston height like you mentioned. The challenge with these systems is the cost, reliability and benefit trade-off, it seems like the value has so far not been enough to be included in production engines.
    We have shown in many of our publications that the opposed piston engine has the ability to minimize the efficiency drop at low loads compared to a 4 stroke engine. This comes from both the ability to preserve high indicated efficiency but to also reduce the pumping work to a very low level and in proportion to the fuel quantity injected. Unlike a 4-stroke engine that has a systematic full rotation dedicated to pumping, the external and independent charging system of the OP engine can be adjusted to only supply the air needed for the fuel quantity needed, not more. We have shown that the OP engine has the greatest advantage over 4 -stroke engines in the low load, low speed region.
    Now about the 50% BTE, we collected brake data on our prototype engines that confirms the capability of the OP engine to achieve this level of performance even without variable compression ratio and still meet the application requirements. So we expect that with variable compression ratio and other technologies that are currently being applied to 4 stroke engines that the opposed piston engine would be well above 50% BTE.
    Thanks for your comment.

  3. I note your engine design maximises the percentage of gas molecules in the exhaust that are one or two atoms – nitrogen and oxygen in the main that retain less heat.

    On a separate point, It appears to me your power unit could be mechanically simplified and its efficiency improved still further if you went in the hybrid direction. The exhaust should fed a turbine-generator that supplies energy only to a battery. The battery could power a turbine-motor unit that compressed air only as needed. The radiator fan could be electric powered, as could the water pump and any other devices so that they only used energy as required. The crankshafts would supply energy only to the vehicle drive system.

    Like the Mercedes F1 car, an electric motor could be placed on the engine side of the clutch to further harvest energy, or supply it, as required.

    FreeValve and others have done away with camshafts, Achates should go the whole hog and strip all accessories off the engine.

    If the Achates engine (power unit) came with its own battery energy storage system that harvested energy from the exhaust turbine and engine/drive train (like Merc F1), and supplied it to engine accessories and the drivetrain (like Merc F1), it would improve the fuel efficiency, increase the power, improve the packaging, reduce engine size, and reduce the emissions and noise still further.

    F1 cars have gone in this direction. The vehicle world is going hybrid due to the efficiencies offered. Achates’ two-strokes, diesel and gasoline, at the heart of a hybrid power unit, look to be the way forward for the internal combustion engine.

    It would be great to see an Achates power unit on the race track, particularly in F1.

  4. Opposed piston technology has been with us since before the design of the Fairbanks Morse, which powered our diesel submarines from the S class after WWI to the auxillary generators on todays Seawolf and Ohio class nuclear submarines with the same opposing piston engine. This engine is remarkable for its reliability at sea, though it failed to catch on in the rail industry.

    Fuel injection technology has been with us since the hot bulb engine evolved into the diesel engine at the turn of the industrial revolution.

    Why has it taken this long to get the same two technologies into gasoline engines on the higway?

    • @killfish – Opposed-piston engines have been around for quote some time, but never achieved the level of success of the four-stroke ICE in vehicle powertrains. There are many reasons for this, but we’ve worked hard to optimize the OP Engine and bring it to market for vehicles. Gasoline Compression Ignition in an internal combustion engine has been under development for quite some time and has faced challenges. We at Achates Power think that our OP Engine is an ideal architecture to address many of these challenges, Fabien and Steve break this down well in the last part of the post. We think that our OP Engine with GCI will be a phenomenal engine, a no-compromises, powerful, cost-effective engine that will power vehicles for a long-time with low emissions and great fuel economy.

  5. You should see new novelty engine which has been designed, more efficient and 50% smaller than above or (conventional) . One of UK universities is developing that engine now. I hope to see the news about it soon…

  6. Truly a revolutionary engine. The GCI engine will next to impossible to beat in efficiency(for a gas engine) for many years to come . With the Cummins partnership & the Army contract along with over 5,000 dyno hours & the major 3 auto giants even talking about this engine, i cant wait to get my hands on one.You can keep everything else. This will be the affordable option for the everyday person or family.

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