The most efficient and maintenance-free engines of all are diesels. Diesels have no ignition systems, carburetors, nor spark plugs.
They burn lots less fuel per horsepower/hour than do gasoline engines, and their exhaust is devoid of many of the toxic emissions of gas engines. Diesels are simple in design and extremely durable. Diesel generators normally run at lower RPM and produce more torque at lower speeds thus increasing engine life and lowering noise. With few exceptions, a properly serviced HD Power Systems Diesel Generator will run for years and years before needing an overhaul.