Diesel engines - healthy RPM range

[mordax]

Hey guys,

Got a question for ya. I have a 93 2.8l Pajero that I've been enjoying since February. Before I bought it, I had test driven a friend's Pajero and Nissan Patrol to see how the whole RHD thing was. I recall when driving his trucks that he mentioned diesel engines like to be rev'd higher than gasoline engines before shifting (his Patrol was a 5pd).

Later on, another friend who drives a B series Landcruiser diesel also mentioned to me that running diesels at low RPMs under load can lead to overheating.

So my question is, what's the healthy range for these Mitsubishi engines?

I have an auto, so I don't as much control over RPMs as I'd like, but I'm still curious.

[Growlerbearnz]

Short answer: 1800-3200rpm is the most efficient range for a diesel.

Slightly longer (ok, it got out of hand) answer: Sounds like your friends are thinking of two different issues (features?) of diesels:

Revving higher before shifting: I think your friend means "it's ok to shift much closer to the redline than you would if it was a gas engine". Diesels only rev to about 4500rpm, so it's common to use most of the rev range. If it's a turbo diesel, not much happens before 2250rpm so you're going to be using the upper half of the rev range a lot more than you would in a gas engine.

Low RPMs under load: this one's a bit more complicated. For a gasoline engine to run properly, the ratio of fuel to air must be within a narrow range. The accelerator pedal operates a butterfly valve (throttle) in the air intake, restricting the amount of air that can get into the engine. The fuel injection computer measures this air, and injects the appropriate amount of fuel to keep the air/fuel ratio constant.

A diesel engine doesn't care about the air/fuel ratio. Diesel engines don't have a throttle, they ingest as much air as they can all the time. The accelerator pedal simply controls how much fuel is injected. Here's where it gets dodgy: At low RPMs under load, it's possible to inject more fuel than the engine has air to burn efficiently. That's when you'll see the black smoke- like driving up a long, steep hill with your foot to the floor. Because the diesel is burning so slowly and inefficiently, it will put a lot more heat into the engine than usual, possibly leading to overheating. The solution is to either ease up on the accelerator pedal, or shift down a gear so the engine's running faster and ingesting more air. There should be no issues running at 4000rpm up a hill if needed.

[mordax]

Awesome. Thanks for the reply!