9913

The Emissions Performance of the Rotapower® Engine

Paul S. Moller, Ph.D. Freedom Motors

Freedom Motors 1855 N 1st St., Suite B Dixon, CA 95620 www.freedom-motors.com

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9913

The Emissions Performance of the Rotapower® Engine

Paul S. Moller, Ph.D. Freedom Motors

ABSTRACT production is different. Maximizing the combustion efficiency minimizes HC production by eliminating This paper discusses the emissions performance regions in the combustion chamber where the fuel of the Rotapower engine. This performance is air mix might get trapped and remain unburned. It compared to other types of engines. The attributes is desirable therefore to minimize low temperature of this engine that make it a clean powerplant are regions in the combustion chamber, which might described. quench the burning process, resulting in unburned fuel. Unburned hydrocarbon production increases ENVIRONMENTAL CONCERNS ABOUT THE rapidly with insufficient oxygen present as does EMISSIONS OF INTERNAL COMBUSTION CO. This can result when operating below the air ENGINES DRIVE THE NEED FOR CLEANER to fuel ratio required to generate a stochiometric POWERPLANTS mixture (the mixture which results in the theoretical amount of available fuel and air necessary for Internal combustion (IC) engines are everywhere, complete combustion (A/F = 14.7)). It is important from the family SUV in America to the two-stroke to get enough turbulence in the combustion motor scooters in Hong Kong to the port able power chamber to ensure an even mixture and eliminate units in developing countries. Around the world local regions where the fuel to air ratio is below legislators are responding to the increasing 14.7. The swishing effect of the rotary engine’s emissions by tightening the allowable limits on IC geometry on the air fuel mixture as its rotor engines. Engine manufacturers are continually approaches top dead center is of great benefit in working to reduce the emissions levels produced achieving an even mixture. by their power plants. Most modifications increase the complexity and the cost of the engine. The Carbon monoxide is also produced when there is Rotapower engine is designed from its inception to insufficient oxygen to fully reduce all the carbon in be a clean burning powerplant. What is most the fuel to CO2. This is especially prevalent when impressive about its very low emission production the engine is running with an air-fuel ratio less is that this is accomplished without additional than 14.7 (a fuel-rich mixture). exhaust after-treatment or the complexities of costly direct fuel injection. Oxides of nitrogen are produced in the environment created by the combustion process EACH EMISSION CONSTITUENT IS FORMED not by the process itself. They are formed in the AND DEALT WITH UNIQUELY wake of the combustion flame in the high temperature burned gases, the higher the burned There are four main emissions components that gas temperature and pressure, the greater the must be considered for any IC engine: formation of these oxides. The most important engine variables affecting NOx emissions are: • HC - unburned hydrocarbons, which can be simplistically thought of as unburned fuel • The air-fuel ratio - NOx production is maxim um • CO - carbon monoxide when operating near the stoichiometric mixture. • NOx - oxides of nitrogen, a grouping of NO and • Spark timing - the point before TDC (top dead NO2 center) at which ignition occurs. Retarding the • Particulate matter - particles suspended in the spark timing, or igniting nearer TDC, reduces exhaust the peak combustion temperature and pressure which results in reduced NOx production. Each of these emission constituents is prod uced • Exhaust gas recirculation (EGR) - recirculating by a different mechanism in the combustion a portion of the exhaust gas into the process and therefore the means to minimize its combustion process increases the bulk mass of the mass to be ignited. This reduces the introduced into the incoming airstream or premixed combustion flame front temperature and in the fuel, also can end up in the exhaust, as consequently the NOx formation. either unburned hydrocarbon or additional particulates. Particulate matter is of concern in engines where the fuel is injected directly into a highly Unburned hydrocarbon emissions from a two - pressurized combustion chamber. This injection stroke engine can be anywhere from 10-100 times causes the fuel to enter the high-pressure region greater than a four-stroke engine of comparable as a mist of droplets as opposed to vapor. Some power. Figure 1 shows the comparative HC carbon portion of these droplets remains unbu rn e d emissions of a traditional two-stroke engine versus because of the finite time required to complete the Rotapower engine in a personal watercraft combustion. The size of the resulting particulates application. An abundant amount of exhaust gas is on the order of 0.1 - 20 microns. Research has recirculation makes two-strokes produce little NOx. shown this process can create particles that are Two-strokes also produce high amounts of CO due carcinogenic. Particle sizes below 5 microns are to the lack of uniformity of the fuel distribution in most worrisome since they are smaller than the the combustion process. The lack of mixture natural filtering mechanism in the human lungs 1. uniformity creates local regions of insufficient oxygen resulting in incomplete combustion of the While different types of fuels have a tendency to fuel. create more of one type of emission than another, all fuels can benefit from engine attributes, which minimize the production of emissions. The Freedom Rotapower engine is based on the Wankel rotary engine, which was first patented in the late 1950’s by Felix Wankel. The Rotapower engine uses the incoming fuel -air charge to cool the rotor, a concept that was put i nto production by Outboard Marine Corporation (OMC) for snowmobiles in the early 70's. In order to minimize rotor housing, seals, rotor and bearing temperatures, this OMC engine was, of necessity, operated at air-fuel ratios well below stoichiometric mixture. Extensive research and proprietary modifications to this engine design by Freedom Motors has allowed the rotor to operate in a lean burn mode, with air-fuel ratios greater than 20. This together with the pre-heated incoming charge inherent in a charge cooled rotary engine, has led Figure 1: Comparison of HC Production to remarkably low production of all four-emission constituents. Manufacturers of two-stroke engines have gone to costly direct fuel injection to help minimize HC ROTAPOWER ENGINE COMBINES THE BEST OF output. While this has cut the HC emissions of the TWO-STROKE POWER WITH FOUR STROKE two-stroke by 50-75% it has aggravated the EMISSIONS PERFORMANCE problem of particulate production. In particular it has introduced the newly recognized problem of In applications where low cost, high power and smaller particulates in the sizes (<5 microns) that lightweight are required, such as in the are carcinogenic. recreational marine industry, two-stroke piston engines are in widespread use. A traditional two - The Rotapower engine operates on the four stroke stroke piston engine eliminates the hardware operating principle using the Otto Cycle. Engines necessary to isolate the intake process from the operating on this principle inherently use fuel more exhaust process, in doing so the cost and weight efficiently. What is unique about the Rotapower of the engine are substantially reduced. However, engine is its ability to use this efficient cycle in a the efficiency of the combustion process is also package that delivers superior power to weight reduced. It has been estimated that 20-30% of the ratio and power to volume ratio at a product cost fuel used in a traditional two-stroke engine is comparable to that of t he two-stroke engine. Table passed through to the exhaust 2. This unburned 1 below outlines these comparisons. fuel having passed through the combustion process becomes a class of compounds known as polycyclic aromatic hydrocarbons, which are known to be carcinogenic, and mutagenic 3. In addition the oil used to lubricate the engine, which is variable port overlap contributes to the very low amounts of observed NOx emissions. Engine Ma x W e ight Volume HP (lbs.) (ft^3) The rotor face temperatures of the charge cooled Rotapower 110 74 1.3 Rotapower engine are significantly higher than its Dual (16"x11"x13") oil cooled rotor alternative. The charge-cooled 2Si 808 100 112 3.4 rotor has combustion surface temperatures that L-1004 (17"x28"x12") are over 400 deg. F hotter than an oil -cooled rotor. These higher temperatures will reduce combustion Table 1: Power Density Comparisons quenching particularly in a higher surface to volume combustion chamber like the rotary engine. THE CHARGE COOLED ROTAPOWER ENGINE The design of the charge-cooled rotor must be IS DESIGNED TO BE A CLEAN BURNER capable of surviving these elevated temperatures. In addition, it must dissipate this heat in a way that While the four-stroke operating principle of the minimizes its transmission to the internal rotor Rotapower engine easily improves upon the bearing. Freedom Motors has developed a emission performance of the less efficient two - proprietary rotor design that has demonstrated the stroke engine, it also has several features that necessary survival and heat transfer make it a low emission engine compared to many characteristics. four stoke piston engines. Rotary engines have historically been thought of as a potentially dirty One of the unique characteristics of the rotary engine when compared to four-stroke piston engine is that while it uses the four-stroke engines. This idea stems from the fact that the operating principle, it does so without the use of combustion chamber in the rotary engine is long valves. In a four-stroke piston engine the valves and thin, with a higher surface to volume ratio. can become very hot when running with lean This geometry is more likely to quench the burning mixtures. In part this is because the fuel in the process, resulting in higher emissions, particularly charge helps cool the edges of the valves during HC, than the lower surface to volume ratio normal operation. With lean mix burn, this cooling geometry found in piston engines. However there source is diminished. More importantly, lean are other attributes of the rotary engine, mixtures burn slower and may still be burning as particularly with charge cooling, which offset the the gases exit past the exhaust valve. W ith no disadvantages caused by the shape of the valves to overheat, the rotary can operate combustion pocket. These include an elevated effectively in lean burn conditions. Additionally, the incoming charge temperature, increased rotor face long slender shape of the combustion chamber temperature, exhaust recirculation, lean burn helps maintain a partially stratified charge. With a capability combined with partial charge richer, more ignitable, mixture at the spark plug, stratification and a high exhaust temperature. this burning mixture can readily ignite the leaner mixture elsewhere. The process of using the incoming charge to cool the rotor has two effects. The first is to fully Figure 2: Rotor Face Temperature (deg. F) vaporize the charge. This is important because a Charge cooled rotor (Freedom) fully vaporized mix is inherently less susceptible to Oil cooled rotor quenching. Complete vaporization also reduces the 348 730 347 338 580 580 possibility of particulate formation in the 374 902 combustion process. The second effect is an increase of the charge temperature itself. The incoming charge is heated to approximately 250 Oil Charge Charge direction direction direction deg. F as compared to near ambient conditions for 369 377 430 620 725 620 two and four stroke piston engines. This increased temperature also reduces the tendency to quench the combustion flame. While the preheating of the charge does decrease the volumetric efficiency of 350 680 the engine, this loss is balanced by the elimination 295 316 550 550 of cooling hardware and the advantage of more 312 630 complete combustion. Source: “Rotary Engine”, by Kenchi Yamamoto Source: Rotapower Dyno Testing Dual Plug Configuration Exhaust gas recirculation is a successful technique used to mitigate NOx. In the Rotapower engine Charge stratification is more easily achieved in a exhaust recirculation is ensured by the increased rotary engine for a number of reasons. The intake overlap of the intake cycle with the exhaust cycle stroke of the rotary engine is 50% longer than the at a higher power setting. Because the Rotapower four-stroke piston engine. This allows greater engine can vary, the intake-exhaust overlap. This latitude in injecting the fuel at the critical portion of the cycle. In addition the geometry of the In diesel engines fuel is required to be directly combustion chamber tends to keep the entire injected into the combustion chamber under high volume of the chamber from mixing, as occurs with pressure. The injection rate control s the the more cylindrical volume of the piston engine. combustion pressure rise during the expansion Therefore despite using a lean overall mixture the stroke. The fuel-air mixture would detonate if the fuel can be port injected early in the cycle and diesel fuel was introduced into the combustion provide a richer mixture near the spark plug for chamber via an intake charge. Research shows good initial combustion. that the highest particulate concentrations in compression-ignited diesels are found in the core The combustion process in the rotary engine region of each fuel spray where the local air/fuel always takes place at the same region of the rotor mixture is running very rich 6. housing. Because the intake charge does not see this high heat flux region there is less chance of Two stroke engines manufacturers have begun pre-ignition or detonation. The combustion surface introducing direct injection models to the market. temperature of the housings can, therefore, be Most of these models use either the Fich t or maintained at a higher temperature by transferring Orbital injection systems. Both systems inject fuel less heat to the cooling system. The charge-cooled under pressure late in the compression cycle rotary engine rejects approximately one-half as where little time exists for achieving uniformity of much heat to the cooling system as a four-stroke the fuel distribution. The Ficht system uses an piston engine does. Consequently much of this electromechanically controlled piston while Orbital heat ends up in the exhaust. This increases the uses compressed air. The mechanism for injection exhaust temperature where it sets up a naturally may vary, however, these engines are still occurring thermal reactor for reduced emissions. susceptible to particulate production due to the This higher quality exhaust energy is available to richer mixture of fuel in the injection region. Toyota be extracted as useful energy by, for example, an has recently reported elevated particulate matter exhaust driven t urbine. emissions (under 2.5 microns in size) in their direct injection engine designs 7. HC and CO emissions can be eliminated from exhaust either catalytically, by the use of chemical The Rotapower engine has a unique advantage reaction, or thermally by burning them with excess over the piston engine in its ability to operate on oxygen. In order to burn the residue hydrocarbons, the traditional four-stroke Otto cycle using diesel the exhaust gases must be resident at a fuel. Since diesel fuel has a very low effective temperature of at least 1100 deg F. To oxidize CO octane rating (40 to 60) it would detonate in a requires a temperature of 1300 deg. F. With piston engine where the fuel/air mixture is exhaust temperatures in 1400-1700 deg. F ranges, homogenous and combustion begins through spark the Rotapower engine has more than sufficient ignition. The lengthy combustion chamber of the thermal energy to minimize these emission Rotapower engine provides as opportunity to sl ow components. the combustion process and allow the Rotapower engine to operate using diesel fuel. This capability THE ROTAPOWER ENGINE DOES NOT was demonstrated under contract to the US Army. PRODUCE THE CARCINOGENIC EMISSIONS OF USING DIRECT FUEL INJECTION ENGINES Without the need for direct injection together with the total vaporization of fuel in the intake charge All IC engine exhausts are a complex mixture of by charge cooling, the Rotapower engine does not inorganic and organic products materials. produce the particulates that are common to However, IC engines, which inject fuel directly into traditional diesel or other direct injection engines. the high pressure of the combustion chamber, The power to weight ratio for the diesel fueled always produce significant particulate emissions. Rotapower engine is many times greater than a Particulate emissions are generally much lower in traditional diesel cycle engine. properly carbureted or port fuel injected engines. Diesel engines produce particulates at a rate about BY COMPARISON, THE ROTAPOWER ENGINE 20 times greater than port inject ed or carbureted IS THE BEST OVERALL PERFORMER IN gasoline engines. Over 90 percent of the mass of MINIMIZING EMISSIONS PRODUCTION these diesel exhaust particles is less than 2.5 WITHOUT AFTER-TREATMENT microns in diameter. Because of their small size, these particles are easily inhaled into the bronchial The engines discussed in this paper utilize and alveolar regions of the lung. Part iculate matter different arrangements for the conversion of fuel has been associated with approximately 50% to into mechanical power. The combustion process of 90% of the mutagenic potency of the whole diesel each is different enough such that the emission exhaust 5. constituents vary from engine to engine. Figure 3 compares the untreated emissions from various engines of similar power output operating with different fuels. The emission production is treatment, doing so increases cost and, eventually, measured in grams per horsepower-hour, which the efficiency of the these subsystems degrades. allows for the accurate comparison of engines of somewhat different sizes. The comparisons are The Rotapower engine is able to minimize the made for untreated exhaust emissions. Some of production of all four major emission constituents the engines can utilize after-treatment to reduce without the additional cost of after-treatment and in these values, some may not. Two-stroke engines, doing so can maintain optimum emissions for example, cannot use the traditional after - performance throughout the engine lifecycle at treatment offered by catalytic converters. The oil in much less cost. the exhaust of a two-stroke engine quickly clogs up the converter. In addition, the low exhaust temperature drastically reduces the efficiency of catalytic type after-treatment. While the four stroke piston engines and diesel engines can use after -

Figure 3: Comparative Emissions Performance 8, 9 ULEV is the California Ultra Low Emission Standard

REFERENCES

1 CARB Staff Report, “Proposed Identification of Diesel Exhaust as a Toxic Air Contaminant”, June 1998, page 4 2 State of California, Air Resources Board, Staff Report, “Public Hearing to Consider Adoption of Emissions Standards and Test Procedures for New 2001 and Later Model Year Spark-Ignition Marine Engines”, October 23, 1998, page 15 3 Hilchey, Tim, “EPA Studies How to Clean Up the Wakes of Motor .” New York Times, May 17, 1994 4 2-Si International Specification Sheet, “808 L-100” 5 CARB Staff Report, “Proposed Identification of Diesel Exhaust as a Toxic Air Contaminant”, June 1998, page 4 6 Heywood, John B., Internal Combustion Engine Fundamentals, page 632 7 CALSTART, “Toyota Introduces Direct-Injection Gas Engine”, News Notes, 7/3/98 8 SAE 931541, “Baseline and Controlled Exhaust Emissions from Off-Highway Vehicle Engines" 9 Burke, Andy, Ph.D., Letter of Witness to Rotapower Engine Testing, Institute of Transportation Studies, UC Davis, 1/10/97

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