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Two-stroke gasoline engines are known to benefit from using in-cylinder fuel injection which improves their ability to meet the strict fuel economy and exhaust emissions requirements. A conventional method of in-cylinder fuel injection involves application of plunger-type positive displacement pumps. Two-stroke engines are usually smaller and lighter than their 4-stroke counterparts of equal power and need a pump that should also be small and light and, preferably, simple in construction. Because a 2-stroke engine fires every crankshaft revolution, its fuel injection pump must run at crankshaft speed (twice the speed of a 4-stroke engine pump). An electronically controlled fuel injection system has been designed to satisfy the needs of a small automotive 2-stroke engine capable of running at speeds of up to 6000 rpm.

The air-hybrid engine absorbs the vehicle kinetic energy during braking, puts it into storage in the form of compressed air, and reuses it to assist in subsequent vehicle acceleration. In contrast to electric hybrid, the air hybrid does not require a second propulsion system. This approach provides a significant improvement in fuel economy without the electric hybrid complexity. The paper explores the fuel economy potential of an air hybrid engine by presenting the modeling results of a 2.5L V6 spark-ignition engine equipped with an electrohydraulic camless valvetrain and used in a 1531 kg passenger car. It describes the engine modifications, thermodynamics of various operating modes and vehicle driving cycle simulation. The air hybrid modeling projected a 64% and 12% of fuel economy improvement over the baseline vehicle in city and highway driving respectively.

Using an electric hybrid leads to a significant improvement in vehicle fuel economy. Unfortunately, it also leads to a substantial increase in cost. Regenerative compression braking offers another way to achieving the same objective without incurring the same cost penalty. With some modifications, the vehicle engine can perform both absorption and recovery of braking energy, using compressed air for energy storage. The process parallels the one employed by electric hybrids, but it requires none of the expensive electric equipment used in hybrid systems. This paper reviews basic principles of regenerative compression braking and its advantages in comparison to electric hybrid systems. It also describes the required changes in engine system and methods of control. Description and mathematical analysis of applicable thermodynamic cycles is given, including computations of cycle efficiencies and indicated mean effective pressures produced during braking and acceleration.