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Fuel consumption at idle

So today Chevy said they are coming out with a cruise diesel. Mazda is supposed to bring the diesel “6” to the us as well… in my current job I drive around 25-30,000 miles a year and work from my car. This means that during the summer I will be sitting with the car at idle with the AC on while doing paperwork, making phone calls and setting appointments. I was figureing I would get a Prius when my buick dies (hopefully in a few years, and no time soon) because you can run the Ac without the gas motor running.

My question is this at idle how much fuel does a diesel motor use at idle??? I once told that big rigs run a gallon an hour at idle, but I have no idea about a smaller car motor. I know letting my Buick idle kills my MPG quickly but it’s a V6, and anything that replaces it will be a 4cyl… How do the new DI gas motors do at idle??

Just curious, thanks guys.

“because you can run the Ac without the gas motor running.” Now I don’t believe that would be a good idea.

You’ll use gas in a Prius sitting there. I do that in my MKZ hybrid, it turns on after the battery’s depleted below a certain level. No free lunch. I imagine a small diesel would use about the same amount of fuel. It’s driving where the vast majority of fuel is used in either case.

@texases, yes, the engine uses more fuel to move the car, but sitting at idle the gas mileage is zero. That really drags down the mpg average.

You must realize that your “engine” is not not very efficient…It would make a pretty good furnace. 70-80% of the BTU’s you buy in the form of diesel fuel or gasoline are used to heat the atmosphere not power the vehicle…So fuel burn at idle is more than you think because only 20% of it is making the crankshaft spin. The rest is making the water boil and the exhaust pipe glow…

Just curious, but why would it be a bad idea? I suppose the motor that runs the compressor could wear out sooner, but that is probably cheaper than idling the gasoline engine. Is there some other problem?

I’m sure you guys are right, but this seems to contradict the advice frequently (and consistently) given here: It does NOT make sense to switch off engine when stopped at traffic light (or coasting downhill).

(But maybe your traffic light stops are shorter than here in the DC suburbs, where the local media have recently hyped the news that we have the worst commute in the nation. For sure, some of my traffic light stops run more than two minutes.)

@MarkM: The motor that runs the compressor will wear at the same rate whether the gas engine is on or not. I can’t imagine that this is any less reliable than a belt-driven compressor such that non-hybrid cars use.

You will use the same amount of energy either way. As others have mentioned, when the battery gets low enough the engine will run to make up the difference. The energy has to come from somewhere. If you are using the A/C, it is using the same amount of power whether you are idling or driving. If you have a full battery when stopping with the A/C on, it will take a little longer for the engine to start, but when you begin driving again, you will have used the same amount of power and the engine will come on sooner. If the battery is nearly depleted when you stop with the A/C on, the gas engine will run, but again, the A/C will have used the same overall amount of energy.

Whether “idling” a Prius or just another compact car with a similar-sized engine, you will still end up using the same amount of energy to run the A/C, assuming the A/C on both vehicles is equivalent in efficiency. The only difference I can see is that you might also be charging the battery and can use that energy when driving.

Diesels of all sizes use less fuel at idle because there is no throttle and no intake vacuum for the engine to work against.
They use so little fuel that keeping the engine warm at idle can be an issue in cold weather.

Fuel burn at idle is very small, whether a diesel or gasoline engine is involved.
With both, the throttle plate is almost fully closed, allowing only a very minimal amount of air to enter, and in both, the pulsewidth of the injector spray will be minimized to maintain a proper ratio with the volume of air being allowed to enter. It takes little energy to turn over a warm engine that’s not connected to a drivetrain. The major loads are really just the alternator and (if you have the AC on), the AC compressor. Unless you’re turning, the PS pump is free flowing, circulating the fluid, so it adds little in the way of load.

As a matter of fact, there’s so little power being applied by the cylinder that the idle has to be “bumped up” a bit to run the AC compressor should you turn it on.

A hybrid setup uses less fuel by shutting the engine down when you stop until the battery drops a bit and starts the engine again, because although ultimately the energy used from the battery by any accessories that happen to be on will need to be replenished by the gas engine, at least little of the electrical power is converted to heat as opposed to the amount of energy that would go out the tailpipe if the gas engine were running. But I’d argue that the differences are minimal, although they’re enough for the industry to be using this approach in their quest for maximum efficieny and minimum emissions.

I’ve never been a diesel fan. Diesels by their very nature have to run at higher temperatures and compression ratios, and that creates more NOx. While I’m of the underatanding that the new systems can scub that out, scrubbing a bad emission out is not as good to me as not creating it to begin with. There are always tradoffs. I also wonder how well these systems will work when they get old.

Guys, Guys… Very often you all get carried on an argument and forget to answer OP’s question…

Gsragtop, idle fuel consumption for most gasoline powered 4cyl cars in good working condition is 0.8 liters per hour with A/C off, and about 1.1 liters per hour with A/C on.

I came up with this figure because my car (as well as some friends 4cyl cars, different brands ) report this consumption on their trip computers.

As a gallon contains 3.78 liters it comes down to about 1/3 of a gallon per hour, on idle, with A/C on.

I don’t own a diesel engine, but I’ll guess their idle fuel consumption will be similar, or slightly less, of their gasoline powered counterparts (with comparable engine displacement).

Tomas, Tomas, you didn;t answer the OP’s question…
He wants to know how much fuel a small DIESEL uses at idle…

Mountainbike, you’re right… I realized it and edited my comment right before seeing your post.

please re-read

I’d guess that it would be a tad more in a diesel, simply because it needs more energy to operate the higher compression cylinders.

Educated guess but the diesel being more efficient it should use less fuel at idle then a gasoline engine. Big rigs parked on the side of the highway for long breaks often leave their motors idling so I can’t see them wasting lots of diesel needlessly.

According to the EPA gasoline engines use between 2 1/2-4 ltrs per hour while a diesel uses between 1-4 Ltrs per hour.

Think about getting an EV (Volt/Leaf)/ToyotasEV. You might as well be plugged-in when you are working in car. Currently free charging at 480v stations- full charge in 20-30mins. I met a guy in a Leaf that did 150miles on freeway and still had a charge.

I’d guess diesels would use less, the higher compression + no throttle makes them more efficient It doesn’t take more energy overall (you get the compression energy back on the combustion stroke).

@Tom Baez I agree that the idle fuel consumption is a non-issue.
If I was in OP’s shoes I would buy a straight diesel.

Taxis idle a lot and many get 600,000 miles before they need an engine overhaul.