In my experience diesels brake better than petrol engines. Maybe it’s because of their longer stroke and better compression. But in any case a manual transmission is best.
That is not expected - while a diesel has more compression, that compression matters zero (other than friction losses - which might be what you are feeling) since all the energy lost in compression is returned in the next part of the cycle when the energy is returned. Unless you have a “jake brakes” which opens the exhaust valve at the top of the compression stroke thus venting the energy to the atmosphere instead of returning it to the power stroke.
A gas engine has a throttle plate which means when the engine is coasting very little air goes into the cylinder and so on the power stroke the low pressure in the cylinder is fighting against the higher air pressure in the engine.
I agree manual transmissions are best, but I’m a minority (I live in the US, other parts of the world prefer manuals) and so I can rarely find them. That said, an automatic in low gear gives plenty of engine braking.
Isn’t it the case that diesel engines continue to compress air while petrols don’t? This plus the significantly longer stroke might account for some of the difference. Yes the air is decompressed right afterwards but significant amounts of work must be lost as heat.
I don’t know what to say. It’s just my personal experience. While driving different cars the diesels always braked much more, and at lower RPMs. Coasting down an incline, put the diesel in 3rd, it will spin up to 2000 then go down to 1600 and stay there, braking the car 10-15 km/h. The equivalent petrol will spin up to 3000 and stay there, and not reduce the car’s speed, just keep it from accelerating. Something like that happened across many different cars and engine displacements. Maybe it’s because the petrols go down to as little as 1L of displacement and 3 cylinders while the smallest diesels are still at 1.6L and 4. Or it’s because the diesels are always turbocharged, while the petrols are sometimes naturally aspirated.
Some is lost as heat, but most is returned. If you are not comparing like sized engines you won’t really be able to tell much. RPMs going up to 3000 is a sign this is working - the more RPM the more resistance.
In my experience diesels brake better than petrol engines. Maybe it’s because of their longer stroke and better compression. But in any case a manual transmission is best.
That is not expected - while a diesel has more compression, that compression matters zero (other than friction losses - which might be what you are feeling) since all the energy lost in compression is returned in the next part of the cycle when the energy is returned. Unless you have a “jake brakes” which opens the exhaust valve at the top of the compression stroke thus venting the energy to the atmosphere instead of returning it to the power stroke.
A gas engine has a throttle plate which means when the engine is coasting very little air goes into the cylinder and so on the power stroke the low pressure in the cylinder is fighting against the higher air pressure in the engine.
I agree manual transmissions are best, but I’m a minority (I live in the US, other parts of the world prefer manuals) and so I can rarely find them. That said, an automatic in low gear gives plenty of engine braking.
Isn’t it the case that diesel engines continue to compress air while petrols don’t? This plus the significantly longer stroke might account for some of the difference. Yes the air is decompressed right afterwards but significant amounts of work must be lost as heat.
I don’t know what to say. It’s just my personal experience. While driving different cars the diesels always braked much more, and at lower RPMs. Coasting down an incline, put the diesel in 3rd, it will spin up to 2000 then go down to 1600 and stay there, braking the car 10-15 km/h. The equivalent petrol will spin up to 3000 and stay there, and not reduce the car’s speed, just keep it from accelerating. Something like that happened across many different cars and engine displacements. Maybe it’s because the petrols go down to as little as 1L of displacement and 3 cylinders while the smallest diesels are still at 1.6L and 4. Or it’s because the diesels are always turbocharged, while the petrols are sometimes naturally aspirated.
Some is lost as heat, but most is returned. If you are not comparing like sized engines you won’t really be able to tell much. RPMs going up to 3000 is a sign this is working - the more RPM the more resistance.