I can't see any reason why an odd number of cylinders is more efficient than an even number. The number of cylinders is important but it does not have to do with odd or even provided that the engine is properly balanced.
The energy in the fuel
is converted into three forms in the engine, useful work, heat into the block, and heat into the exhaust
. The goal is obviously to make the useful work the highest percentage of this total possible. Changing the heat out the exhaust
is possible but really comes down to a well designed combustion process in the cylinder. However, it is possible to change the heat that is lost
to the block. The important ratio here is surface area of cylinder/volume of cylinder. The larger the cylinder is, the lower this ratio is meaning that the engine is more efficient. For a given displacement
, the only way to get a larger cylinder is to decrease the number of them. This is a major reason why large diesels such as those in ships can achieve over 50% efficiency. However, this extra efficiency can only be realized if the combustion process in the cylinder is optimal which requires adequate airflow, fuel atomization, etc.
Changing the number of cylinders also needs to affect the bore or stroke or both. This will mean that the engine has different hp, torque, and rpm
characteristics and has some efficiency effects as well.
While I do not have much experience with Isuzu engines, all of it has been quite good.