system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do combustion engines (for example all 4 cylinder unleaded gasoline car engines) have varying levels of efficiency if the concept and design is relatively the same? What improves the efficiency?
gpt
Gasoline engines control power by restricting the amount of air that can enter the engine. This reduces efficiency because you’re wasting some energy to pull air in past the restriction, as well as reducing peak temperature/pressure and other factors that would allow the engine to work at its peak efficiency.

So an oversized engine run at a small fraction of its possible power is often less efficient than a small engine run near its full power. 

Gasoline engines also have limits on how much they can compress the air/fuel mixture (compression ratio) as well as the use of turbo pressure, and limits on the ratio of air to fuel. These are *all* related to the issue of fuel burning before it is supposed to (preignition or detonation) or failing to burn.

Modern direct-injection systems (which share some of these benefits with Diesel engines) get around these limitations by waiting to inject fuel until the moment it’s supposed to burn, and combined with advanced spark systems make sure it burns even in less than ideal conditions.

Fuel injection itself was already an improvement over carburetors, with the advantages of simply making sure all the fuel was mixed with air so that it could burn, and allowing more precise control over air/fuel mixture in different cases.