Market forces require that, to be successful and survive, consumer products must be designed to satisfy a selection of requirements imposed by the consumer. Over time and competition, natural selection refines the design so that it is either better optimized and competes or it falls into the dustbin of history.

Engines, while simple in concept, are spectacularly diverse in implementation. When you think "engine" think "mammal" and then imagine all the different species of mammals. Most all are successfully reproducing. Each is optimized for a particular set of conditions that allows them to dominate in a specific niche. The same is true for engines. Every mass produced engine is optimized for a specific set of requirements and no two requirement sets are the same.  Taking an engine from one application and inserting it in another results, de facto, in a sub optimal situation. You could couple the 350 Hp diesel engine from my truck to a weed whacker and it would indeed whack weeds but the portability requirement would not be met.

Aircraft engines are designed with the primary requirements of  light weight, low speed, high continuos percentage power, high reliability and a narrow power band. Think "Draft Horse".

Automotive engines are designed for low cost, high speed, low emissions, high peak power and  wide torque range. Think "Cheetah".

What would happen if you hitched four Cheetas to your plow?

Modern automobile engines have benefited in billions of dollars in development with the ONLY goal of making them more attractive to automobile buyers. The corollary is that all that effort  has been expended to make automobile engines that deliberately do not directly satisfy the requirements of aircraft.

The landscape is littered, both figuratively and literally, with well meaning, would be aircraft engine designers who share a common inability to learn from history.  Perhaps driven by the false assumption that desirable automobile engine characteristics can be selectively transplanted into an aircraft with the simple transposition of hardware. The problem, of course is that all the "not desirable" for aircraft characteristics are part of the bargain. Eating the brains of your enemy won't give you their wisdom but it may give you Mad Cow disease.

People who know enough to design an aircraft engine are wise enough not to.

Brent Regan