by Fabien Redon and Steve Ciatti
Vice President, Technology Development, Achates Power; Principle Mechanical Engineer, Argonne National Laboratory
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.
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:
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.
The 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.
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.
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!