why do some engine maintenance cycles run off hours, like generators, but others off miles, like cars? (Besides the obvious)


why do some engine maintenance cycles run off hours, like generators, but others off miles, like cars? (Besides the obvious)

In: Engineering

Hours is a more accurate measurement. I think cars go by miles because most people are commuting and the starting, stopping, and shifting gears all the time puts more stress on the engine than just contralto running, like, say a generator, which doesn’t change gears. The mileage is also useful in tracking for other systems like the transmission, which doesn’t suffer wear when the vehicle is on, but in park. Also, cars are subjected to different environments, which have different effects on the engine and vehicle as a whole. Mileage vs engine hours is just a way to try to compensate for all of these factors.

Hours is probably the best indicator unless you want to use a much more complex system.

Cars typically use mileage instead because traditionally it was quite easy to track with just a simple mechanical setup, and for the average car there’s going to be a reasonably good correlation between miles and hours (though there are obviously exceptions). It was cheap and easy to measure and it was good enough, so that’s what the car industry stuck with, at least on the driver-facing side.

Internally modern cars will actually track a lot more. Manufacturers will sometimes deny warranty claims if they see that a car or truck has an excessive number of idle hours on it, even if the mileage is still inside the warranty limit.

The oil life monitors on cars with flexible oil change intervals will actually take a lot of factors into account, like how hard the engine has had to work or how many cold starts have accumulated since the last oil change. But even though the engine computer is keeping track of those numbers, manufacturers only expose the mileage since that’s what people are familiar with.

Hours are used for things like tractors and construction equipment. A tractor could spend all day working on a farm and only drive a few miles. So measuring miles isn’t a great way to tell you the amount of wear and tear it has gotten. Since cars are designed to cover distance, measuring distance is a good way to monitor them. A lot of tractors and equipment also has PTOs. These are shafts that the engine can use to run all sorts of devices, like earth moving equipment. Some of these might have you using the engine without the vehicle moving anywhere.

Generators don’t travel. Therefore the only measurement you have to estimate the wear on the engine is time running. Generators also have a constant RPM, whereas cars are variable.

Cars travel, and because they are on the ground we can measure the distance they do. This is a more accurate measure for cars because engine speed is loosely tied to distance traveled. What we really care about in both cases is how many times the crank has turned. Most of the time your car is going to be cruising at 2,000 – 3,000 RPM, so it’s reasonable to extrapolate the number of revolutions based on the distance traveled, with some wiggle room.

Frankly I don’t like either measurement because all that really matters is how many times the engine completed a revolution. That being said, measuring engine revolutions would be counted in the billions over the life of the engine, so I can see why it wasn’t practical before digital sensors became a thing. That does make me wonder, however, if modern cars with oil life meters actually run these calculations instead of using hours or miles. Interesting to think about!

The reason is actually pretty simple – “What measurement best estimates the wear?”. If we look at a forklift(tracked by hours) vs a car (tracked by KMs). The forklift is generally not covering great distances and a lot of the stress to the engine is coming from running the hydraulics (which means the tires dont need to be moving to be wearing the engine). The car on the other hand stresses the engine through running primarily the drive train, and so distance traveled is a better measure than the number of hours the engine was running.

I have never actually never worked with maintenance schedules for long haul trucks, but I imagine they would have a combination of the two (5,000 KMs or 250 hrs, whichever hits first), since they idle enough to require taking that time into account on the wear.

The reason to use miles over hours in cars has less to do with the engine (where the ultimate in metrics for service scheduling would be total rotations of the crankshaft and would be a relatively high number) and more to do with all the other stuff in the car, particularly tires and brakes. Tires only experience significant wear when you’re driving on them, and brakes only experience significant wear when you’re driving and have to stop.

You might be able to go longer without some services if you counted hours or revs of the crankshaft, but having one metric to base all your different services around is an easier system.

Generators run at a constant speed, cars do not. Also for cars miles are easier to track than hours.