I drive a car with a manual transmission down a long hill. Putting it in second gear lets me maintain a safe speed with no use of the brakes - in this condition the engine spins at around 4000 rpm. This is obviously kinder on the brakes than riding them all the way down in a higher gear.
Since the engine speed is considerably higher than idle, does it consume more fuel than it would at idle, even though it isn’t producing power?
I wouldn’t think it uses any more gas. You’re basically using the compression cycle of the engine to slow the vehicle. In this case I’d think it’s just compressing mostly air, though, since you aren’t pressing the accelerator pedal. I am not a mechanic.
If you’re not pressing on the gas, it’s not using any more gas than it would need to idle (more or less). The gas makes the engine go faster but making the engine go faster doesn’t use gas.
BTW the increased engine wear in the long term will be more expensive than the brake wear that you are saving.
You’ll burn a little bit more gas than at idle since the carb output is tied to airflow through the engine.
But not as much as at cruise since the throttle plates will be closed.
And there should be no more engine wear. The oil is still flowing. . .
In a perfect world, but in the real world there will be very, very tiny unlubricated areas, dirt making oil clump, etc
Note that in extreme cases, though (e.g. driving down a loooong winding mountain road) you should absolutely use engine braking, whether on a manual or an automatic. This is to prevent your brakes from overheating and becoming ineffective, which is obviously a safety hazard in such situations.
What? Are you claiming that driving down hill causes more wear? :dubious:
I’m guessing that if the engine revs up when you try to slow down, probably by down-shifting, then it will use more gas. I’m not sure, but I’m guessing that some fuel is drawn into the engine due to the increase in vacuum.
Hey, this one’s easy. Assuming the car in question is relatively new, you’re actually not using ANY gas, since the computer shuts off the fuel injectors while coasting in gear at a speed that keeps the engine spinning faster than idle speed.
The answer for an older car, particularly a carbureted one, is somewhat more complicated. I suspect that the fuel consumption is probably somewhat higher since the engine generates more vacuum, but since fuel consumption is so low at idle anyways it’s probably not an appreciable difference.
Also, CookingWithGas, there is essentially no measurable extra engine wear from engine braking and using your brakes on a long downhill grade will drastically shorten the life of the brake pads and rotors. Not to mention the possiblity of the brakes overheating and failing-- a new engine is cheaper than a new set of teeth or worse!
Won’t the engine’s computer still try to maintain a perfect air/fuel mixture? Shutting off the injectors would cause a *lean *condition. . .
No, Im suggesting that driving down a hill, using engine breaking (resulting in 4,000 RPMs) instead of breaks (resulting in whatever RPMs your engine idles at) will cause more engine wear.
Why? Because this is not a perfect world, and there will be areas of poor lubrication that will be worn more by the higher RPMs than they would’ve from the lower RPMs.
Such as?
(totally not trying to be a smartass)
GreasyJack speaks the truth - try it if you’ve got a relatively modern car with manual transmission. If you’ve got a mpg meter onboard, pop it into instantaneous mode, roll down a hill in gear and with your foot off the throttle and watch it max out. You might need a long downhill section to verify this as the mpg meter isn’t truly instantaneous, it works on a rolling average over a sampling window of a minute or so. My mpg meter tops out at 999 mpg (not enough digits for infinity), and at the other end of the scale it blanks out below 20 mph as it doesn’t want me knowing the high fuel consumption at these low speeds.
It’s not a lean condition, it’s no condition at all. The pistons just go up and down and no combustion occurs.
To be fair (and before the nitpicking begins), this isn’t a universal feature-- on older FI’ed cars and possibly even some new ones, the computer does just continue to maintain the air/fuel ratio, in which case you have a similar situation as to a carbureted car where since there’s more airflow there’s more fuel usage, but not much. Given how advanced most FI systems are on cars these days and the emphasis on fuel economy, I’d be suprised if there are that many newer cars that don’t shut off the fuel while coasting.
Hmm, interesting. The MPG gauges I’m familliar with are nothing more than vacuum meters.
Even so, does that indicate that the injectors are actually turned off?
I doubt that they get turned off, since that would stall the car (if it wasn’t moving). Instead, they just get set to the lowest idle injection.
Such as what?
I don’t understand the question.
I agree that it’s better to stay in a low gear than to brake down a long downhill. But I don’t understand how there can be “no measurable extra engine wear from engine braking.” Engines do wear. Wear is most on a cold start but there is certainly wear during normal warmed-up engine operation, isn’t there? And that wear would be proportional to the number of revolutions turned. Engine braking means more revolutions. So where am I wrong?
No, in modern cars, they for real get turned off. All the way off. And they’ll stay that way as long as the momentum of the car is keeping the RPMs significantly above idle.
You’re probably right on some theoretical level, but I would be willing to say that on a modern engine, for all practical purposes there is no wear when the engine is warmed up and operating normally. With modern engines running modern oils, wear in general has simply become a non-issue-- the only way engines ever wear out is if they’re abused or neglected. If you do a basic level of maintainence a modern engine, it will be long outlast the other components on the car.
So, with that in mind, I think that the cost of reduced brake component life would exceed by many orders of magnitude the costs associated with increased engine wear (assuming they were even quantifiable). Even if we were to suppose that the increased wear from a lifetime of engine braking shaved ten or twenty thousand miles off the life of the engine, failing to do so will shave a lot of miles off of every set of brakes you buy and that will add up much faster.
All the computer needs to know to prevent the car from stalling is whether it’s in gear or not and if it’s moving fast enough. Cars have had vehicle speed sensors for a long time, so the only real hurtle to fuel-off coasting was that most older cars didn’t have a sensor that told the computer whether it was in gear or not. These days though, most automatics are computer controlled so that last piece of information is readily available.