I want to make a car engine whose efficiency is slightly double than the present car engines and which was an eco-friendly engine.
i think it is little impossible
we have to see various parameter ,such as design of intake and outake design of fuel injector etc
and you can increase the efficiency by 4to 5 percent not double
Yup efficiency of an engine has a upper limit. However we are far away from reaching that. Go for contactless actuations like camless drives. Cause friction does have a lot of impact.