The throttle will adjust the air-fuel ratio. For the diesel engine, the combustion takes place around individual droplets of the fuel spray, therefore the output is related to the amount of fuel injected, and the air flow is not controlled. Fig 2 shows that the efficiency increases with the power for both engines, however the diesel engine reaches higher efficiency than the petrol engine. This is because, for the diesel engine, the input of the fuel is in the form of fuel spray.
The compression of the fuel and air is avoided. This will make the maximum cylinder volume to minimum larger, and leads to higher efficiency. So, the petrol engine should be used in light vehicles because it is lighter than the diesel engine and it can reach higher power than the diesel engine which produces higher speed of the cars. And, the diesel engine should be used for the heavy vehicles, since it has higher efficiency. The heavy cars need to do more work, so the efficiency is more important to them.