Throttleless Premixed-Charge Engines

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1.  Ronney, P. D., Shoda, M., Waida, S. T., Durbin, E. J., “Throttleless Premixed-Charge Engines: Concept and Experiment,” Journal of Automobile Engineering, (Proceedings of the Institution of Mechanical Engineers, Part D), Vol. 208, pp. 13-24 (1994).

2. Durbin, E. J., Ronney, P. D., "Method and Apparatus For Force or Torque Control of a Combustion Engine," U.S. Patent No. 5,184,592, Feb. 9, 1993.

3. Ronney, P. D., Shoda, M., Waida, S. T., Westbrook, C. K., Pitz, W. J., “Knock Characteristics of Liquid and Gaseous Fuels in Lean Mixtures,” Transactions of the Society of Automotive Engineers, Vol. 100, Part 4, pp. 557-568 (1992). Also SAE Paper No. 912311, 1991.

Why throttleless premixed-charge engines?

Conventional premixed-charge spark-ignition engines employ a throttle to reduce power and torque when demand is low by reducing the pressure of the combustible mixture drawn into the cylinder. This results in the well-known “throttling loss.” Under typical highway cruising conditions, this loss is typically 15% or more of the otherwise available power output of the engine. This loss leads to reduced fuel economy and increased pollutant emissions.

We have developed an engine control concept, called the Throttleless Premixed Charge Engine (TPCE), which provides the necessary range of power and torque adjustment without throttling by using a combination of lean mixture and intake air preheat to adjust torque. Higher intake temperatures reduce the air density and thus power and torque. Leaner mixtures also reduce power and torque, and the intake air preheat substantially reduces the lean misfire limit. Thus in the TPCE concept the synergistic use of preheating and lean mixtures is essential; neither technique individually provides a sufficient range of power and torque adjustment for use in practical motor vehicles.

A detailed discussion of the concept, testing and implementation of the TPCE engine is given in our technical paper [1] that won the British Institution of Mechanical Engineers Starley Premium Award for the best paper published in the Journal of Automobile Engineering in 1994. Also, we have been granted a U.S. patent (No. 5,184,592) for the concept. A brief summary of our work is presented here. Our current work, supported by the South Coast Air Quality Management District and the National Center for Metropolitan Research (METRANS), is discussed at the bottom of this page.

Applications of the Throttleless Premixed Charge Engine concept

The TPCE concept is ideal for vehicle applications because vehicles are constantly changing load and speed, and are only infrequently operated at wide-open throttle. In this sense the TPCE concept provides many of the best aspects of premixed-charge, spark-ignition engines (fast response time, high power to weight ratio, and negligible particulate emissions) with the best aspect of nonpremixed-charge compression-ignition (Diesel-type) engines (higher part-load thermal efficiency due to lean operation without a pressure-reducing throttle). For these reasons the TPCE concept is equally applicable to light-duty vehicles as well as heavy-duty trucks and buses.

The TPCE system is easy to retrofit to existing engines because only a change of the intake, exhaust, and engine control systems is required for the basic installation. Of course, engines specifically designed for the TPCE application would realize more substantial performance gains.

Summary of results to date

We conducted experiments to test the TPCE concept using 4-cylinder production engines with natural gas fuel. As shown in Figure 1, these tests showed the theoretically expected level of improvement in brake thermal efficiency (up to 16% compared to the same engine operated using conventional throttle control at same power and engine speed) due to the absence of throttling loses. Also, because of lean operation, greatly reduced NOx emissions are obtained - typically 10 times lower than the same engine operated using conventional throttle control at same power and engine speed (see Figure 2). The observed NOx level of less than 0.8 grams of NOx per kW-hr (0.6 grams per hp-hr) at moderate and light loads corresponds to less than 0.2 grams per mile for a typical 15 hp road load at 55 mi/hr – without the use of a reducing catalytic converter for NOx removal. This emission level is half of the 2001 California standard, and equal to the 1998 “Clean Fleet” standard required for 30% of new vehicles used by centrally-fuelled fleets in cities with poor air quality. CO and unburned hydrocarbon emissions in TPCE engines were found to be comparable to throttled engines, thus only inexpensive oxidizing catalysts for CO and UHC are needed for TPCE engines. Torque control via exhaust gas recirculation (EGR) was also tested and found to have similar NOx characteristics but vastly inferior thermal efficiency and torque adjustment range.

Figure 1. Comparison of brake thermal efficiency of throttled and throttleless (TPCE) engines using natural gas fuel. Experiments employed a 4-cylinder 2.5 liter General Motors LX8 engine.

Figure 2. Comparison of measured brake specific NOx (BSNOx) emissions of throttled and throttleless (TPCE) engines using natural gas fuel. Note logarithmic scale on BSNOx.

Current efforts

In the TPCE and throttled engine experiments described above, the engine was operated in a manner compatible with vehicle applications in all aspects except one: for experimental convenience the air was preheated using an electrical heater (see Figure 3), whereas in a practical application the air preheat must of course be accomplished via heat exchange with the exhaust gas. A practical means of providing rapidly controllable, variable air preheat is proposed for assessment in this work (see Figure 4.) In this scheme a diverter valve and branched intake duct, one branch passing through the heat exchanger and the other bypassing the heat exchanger, is used. By adjusting the diverter valve position, cold mixture is always available without delay when the torque demand (commanded by the driver or cruise control system) increases suddenly. Thus, good dynamic performance characteristic of throttled engines is maintained despite the unavoidable thermal lag associated with heat exchangers. An additional advantage of this scheme is that only one additional moving part (the diverter valve) is needed. A simple computer analysis of a concentric tube-bundle counterflow heat exchanger indicates that an exchanger composed of 50 tubes, each 4 millimeters in diameter and 40 cm long would suffice for a 2.5 liter test engine. The overall size of the heat exchanger would be about 5 cm in diameter and 40 cm long, which is readily accommodated under the hood of modern vehicles. The pressure drop across the exchanger would be less than 0.05 atm, that is, much less than that due to throttling. Consequently, in the proposed study the viability of the heat exchanger / branched manifold / diverter valve scheme for control of TPCE engines will be assessed with respect to emissions performance, thermal efficiency, and dynamic response.

Figure 3. Schematic diagram of existing engine test facility at USC using electrical preheat of intake air (gaseous fuel implementation shown; liquid fuel configuration also available.)

Figure 4. Proposed implementation of TPCE using heat exchanger and diverter valve to control inlet temperature and thus torque output. (Carbureted gaseous fuel system shown; fuel injection system to be used for liquid fuels).

In the TPCE system, three engine operating parameters need to be controlled: mixture ratio, intake temperature and ignition timing. This wide control parameter space allows the engine performance to be optimized for lowest emissions while maintaining good fuel economy. For example, lean operation reduces NOx emissions but may lead to unacceptably poor efficiency due to misfire if the engine is operated too lean. Also, elevated intake temperatures are advantageous because of the wider flammability limits and lower intake mixture density this entails, but too high intake temperatures will result in destructive engine knock (see issue 2 below). In the proposed work we will determine the proper program of control parameters for both static and dynamic engine operation, including transient conditions such as acceleration and deceleration.

Since the TPCE concept requires the use of lean mixtures and preheating of the intake air, TPCE engines are in general limited by lean-limit fuel performance and engine knock (sometimes called “autoignition,” “detonation” or “pinging”) which is more prevalent at elevated intake temperatures [2]. For these reasons conventional fuels such as gasoline, which have relatively poor lean-limit performance and knock characteristics, do not perform well in TPCE engines [1]. However, by using natural gas fuel, which has excellent lean-limit performance and anti-knock properties even at elevated temperatures, the required range of torque adjustment was demonstrated [1]. Somewhat unexpectedly, lean mixtures exhibited excellent resistance to engine knock compared to stoichiometric mixtures. This is significant because the intake charge preheating used in the TPCE concept might have worsened knock problems, but for lean mixtures our research has shown that this is not the case. An important implication of these results is that optimal fuels for TPCE engines may be different from those of conventional stoichiometric-burning throttled engines.

As a results of these findings, we will examine the performance of other alternative fuels such as methanol, ethanol and hydrogen in TPCE engines. These fuels also have greatly enhanced lean limit and/or anti-knock performance relative to gasoline and thus are likely candidates for TPCE operation. In particular, methanol and ethanol have much higher octane ratings than gasoline blends, and hydrogen has the lowest lean misfire limit of any fuel. Consequently, the TPCE studies will be extended to evaluate the performance of these alternative fuels and fuel blends. These results will be compared to the performance of natural gas fuel, which has already proven to be an excellent choice for TPCE engines. This work is being supported by the South Coast Air Quality Management District and the National Center for Metropolitan Research (METRANS).