1. Field of the Invention
The present invention relates to supercharged internal combustion engines. More specifically, the present invention is directed to a turbocharged-intercooled engine provided with an intake-manifold charge of temperature lower than the ambient air at high compression which is mixed with fuel forming a low-equivalence-ratio mixture resulting in a burned charge of low peak-cylinder-temperature.
2. Background of the Prior Art
H.R. Ricardo stated, "The piston engine is eminently suitable to deal with relatively small volumes at high pressure and temperature and the turbine, by virtue of its high mechanical efficiency and large flow areas, to deal with large volumes at low pressures. Clearly the logical development is to combine the two in series to form a compound unit" (Smith 1955:279-280). He envisaged an engineering system, not as a modification of the piston engine, but as a new rational whole with the compelling logic of resulting mechanical (gas exchanging) advantage and thermodynamic advantage. This thermodynamic advantage will be further enhanced by adding intercooling to the combined system.
The efficient gas-exchanging advantage of turbocharged engines produces the desired power boosting. However, the theoretical thermodynamic advantage of turbocharged engines and turbocharged-intercooled engines is not fully realized for engines that are required to operate over a wide range of speed and load for the following reason: a turbocharger unit and a piston-engine unit are mechanical devices of complete different characteristics. While they can be matched at a given operation point, they are poorly matched away from this specific operation point. For example, when a turbocharger, which is matched with a piston unit for good torque-back-up, is operated at high engine speed and load, its turbine pressure ratio is much higher than the required value. This results in excessive available energy in the exhaust charge driving the turbine. One common solution is bypassing some of the exhaust charge through a "waste gate." This way of forcing the two units to operate together over a wide range of speed and load by a highly dissipative process is an unsatisfactory engineering solution, which counteracts the thermodynamic advantage of turbocharging. In fact, specific fuel consumption of turbocharged gasoline engine is often worse than an equivalent naturally-aspirated engine at high speed and load, conditions under which waste gate is in effect.
Turbo-compound engines may be considered to be an engineering solution to reducing the dissipative loss of waste gate operation by harnessing exhaust gas energy in a power-turbine. Unfortunately, this solution still encounters the mismatching difficulty of two components in a complex system--the two components here being the power turbine unit and the piston unit. Complex transmission between them is required in order to achieve the expected gain in efficiency. This results in a system that is too complex to be commercially successful. The full realization of the considerable potential of turbocharged engines in both mechanical advantage and thermodynamic advantage thus calls for a simple and effective solution to the matching problem.
Another potential advantage of the application of turbocharging, which was not directly pointed out by Ricardo, is the advantage of low operating-temperature. The benefit of lowering operating temperature is considerable, including reduction in NO.sub.x production, engine operating away from smoke limit, durability and ease of maintenance. Lower operation temperature can be achieved by reducing the fuel-air ratio or the equivalence ratio in combination with increasing supercharging pressure ratio, so that brake mean engine pressure (bmep) remains unchanged. The challenge is in lowering operating temperature while maintaining bmep without sacrificing thermal efficiency. My investigation has uncovered a design-parameter combination for a turbocharged engine such that its bmep remains unchanged (under constant peak-cylinder-pressure constraint), its peak cylinder temperature is reduced significantly, over 300C, and its thermal efficiency will increase moderately.
Since the Otto prototype four-stroke engine was first used in 1876, significant improvement in engine performance has been brought about according to the Carnot-Otto-Diesel paradigm: theoretical thermal efficiency of combustion engines increases with increasing operating pressure (mechanical load) and operating temperature (thermal load). Operating pressure and operating temperature do not vary independently. Improvement in engine performance requires the simultaneous advances in mechanical load limit (cylinder pressure and turbo speed) and thermal load limit (exhaust temperature, and smoke limit in the case for diesel engines). This performance improvement has slowed in recent years due to the difficulty of raising thermal load. The present invention suggests a revision of the Carnot-Otto-Diesel paradigm: thermal load is removed as a limiting factor in improving engine performance. With the possibility of keeping operating temperature from rising (with pressure) and compression ratio kept at moderate level due to high a (supercharging parameter), it is once again attractive and viable to consider increasing peak cylinder pressure for performance gain. The onward performance improvement in thermal efficiency and power output of internal combustion engines, since their invention by Otto and Diesel, can be resumed.