[quote=“jcd, post:9086, topic:42”][quote=“Dlightfull, post:9080, topic:42”]have you ever driven a car? in practice, not in theory, people rarely drive gasoline cars at more than 3k rpm, because it makes a hell of a lot more noise. When you get to 2,5k-3k rpm you switch gear (i hope i said it right, english is not my native language).
For diesel cars, it’s the same, you usually want to switch gear at 3k rpm, not more.
I am talking normal driving, not racing.
While it’s true that gasoline engines have a higher limit on the rpm (about 7-8k iirc) than diesel engines (5-6k rpm? haven’t driven a diesel car for some time) the normal usage doesn’t go often above 3k rpm.[/quote]
For me normal usage goes easily to 4,5k rpm in my gasoline engine car. That’s max gear and 130-140km/h
Also for that extra power when i need to overtake a car or in steep angles.
Of course, i strive to work mostly at ~2,5k rpm with the highest possible gear, as i found that’s the best fuel economy for my car.
I have not driven a diesel at all, but it would always be at a slower rpm (in a similar situation) because of physics:
In a gasoline engine, the fuel comes premixed into the cylinder and then the spark plug creates the ignition. So there is no limit to rpm, other than the mechanical and thermal stress to the engine.
But for the diesel engine, pure fuel gets injected into the cylinder at a vey high pressure, and its GOT TO mix very well with the already existing air over there in order to self-combust. This happens in a very short period of time, and in order to achieve it, you have to allow for lower rpms, as rpm directly affects the available time.
Failure to achieve a good fuel-to-air mix means that there will be unburnt fuel in the cylinder that will eventually combust in a destructive, for the engine, way (this will not be instant, rather this frequently happening will destroy the engine)
There are many designs for diesels where the mixing happens faster (by manipulating the fuel injection and cylinder geometry e.g. creating a ‘swirling’ effect), but this has problems in that it will reduce the fuel economy (more energy will be lost mixing the fuel), requiring even greater pressures in order to achieve the same fuel economy.
Anyway, point is that this inherent feature of diesels will ALWAYS make them slower than equivalent gasoline engines in an equivalent situation.
I mean, i have been in diesel cars as passenger, where i think i noticed that usual low-load rpm (when moving) was 1000-1500 while for the gasolines it is over 2000. High load is 3-3,5k for diesels and 5-6k for gasolines. What does this say?[/quote]
look, i have both diesel and gasoline cars and i can tell you for sure that the rpm difference is very low. When the car is on stand by, engine running, but in neutral gear, both of them work at about 700-1200 rpm, depending on engine. And when driving, both of them have a normal usage at 2k-2,5k rpm, as you said yourself, the most economic way of driving it.
At 3k rpm, diesel makes a lot more noise than a gasoline engine.
I have no clue about the theory behind it, but i am telling you how it is in practice.
