any builders of homebuilt aircraft look at the cost of certified aircraft engines and believe they can save money by using a converted automobile engine. There have been many successful installations of such engines, but the builder must be aware of the engineering aspects of engine characteristics and systems.
The history of engineering is full of examples of experimenters confounding the theorists and vice versa, but in general, successful engineering depends on a combination of knowledge, analysis, experimentation and test.
A lack of knowledge and analysis means more experimentation and test. In aircraft powerplant design, this can lead to forced landings or worse. This two-part article will try to impart some basic engineering knowledge and analysis for those who wish to experiment with converted automobile engines.
An engine's end use determines its characteristics. Over the last century, the internal combustion aircraft and automobile gasoline engines have evolved to have the characteristics found in Table 1.
The difference between maximum rated power and normal rated power is extremely important to this discussion as it directly affects reliability and TBO.
Automobile engine power is quoted as maximum rated power. The typical road-load power (rolling resistance plus aerodynamic drag) required of an automobile engine is only about 20 hp at highway speeds.
The maximum rated power gives an indication of the ultimate performance of the vehicle. However, when automobile manufacturers offer the same engines as industrial engines, they are rated at normal rated power. Contrasting this, in an aircraft, we expect TBO of 1500-2000 hours in an engine that delivers 55%-100% of rated power, averaging about 75% of rated power. To illustrate the point, consider the ratings of the same V8 engine shown in Table 2.
An automobile engine produces energy, which the pistons or rotor convert into torque for transmission to the wheels. Though torque is greatest at high speeds, the amount of torque needed to operate a car does not always vary proportionately with speed. At moderate speeds and on level roads, the engine does not need to provide a great deal of torque. But when the car is starting, or climbing a steep hill, it is important that the engine supply enough torque to keep the car running; otherwise it will stall. To allocate torque and speed appropriately, the engine may decrease or increase the number of revolutions per minute to which the rotors are subjected.
In summary, an aircraft engine must be reliable and light with small bulk and good power-to-weight ratio. Successfully converting an automobile engine for aircraft use requires a good deal of engineering design, analysis and test. The hope is that the information in this two-part series serves as a guide for those who wish to embark on the quest of lower-cost aircraft engines.