ArkiStan

This question popped into my head a couple weeks ago:

1. Situation

I had just finished a meal at a restaurant located at the top of a mountain. I was on my way back down the side of the curvy mountain road when I realized I was dangerously low on gas. The nearest gas station was in town at the bottom of the hill, and that was at least 15-20 miles away. If my car died in the middle of the mountain side, I could hitch a ride to get gas, but then I'd be late for my next appointment.


2. Question

Usually when I must go down long down-hill distances, I stay in low gear (1st or 2nd, depending on the slope) since relying solely on my brakes will result in excessive wear. So I'm revving relatively high(around 3,000 RPMs), and I wonder, The engines revving but is it actually doing work? Is it consuming as much gas as if I were running 3,000 RPMs on a regular level street?

This was important because I was so low on gas.

johnnymk

Since the engine is acting like a brake, more fuel would be consumed per unit time. If I remember correctly, on a level road, it only takes 10-20 horsepower to keep a low weight vehicle at a constsnt highway speed. I am not sure how many horsepower is required for the car in the situation you described, but it has to be greater than what is required on a level road.

I am surprised that in the situation you were in that you didn't put the tranny in neutral and try to coast for as long as possible.

InfiniteNothing

You should avoid running low on gas anyways. Your fuel pump uses the gas tank as a heat dump and if you don't have alot of gas the gas can't absorb as much head and you wear down the pump alot quicker. But that's besides the point.

Question 1. It depends. If it were a hybrid it would be recharging the batteries when you went down hill essentially creating gas (Well, sorta). Assuming it's not and you are off the gas pedal, I can't see any reason for the car use any gas. Normally on a flat plane you need a certain amount of gas to fight friction (that is the 10-20HP mentioned above) and maintain a certain velocity. In your case, gravity (instead of gas) powers your car and provides the force to make your pistons rev higher. Friction provides the force to slow you down. You don't even need the small amount of gas you'd normally need to keep your engine from stalling.

Question 2. No, it is not doing work. Gravity is doing work.

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InfiniteNothing

That and idling takes gas.

djradam

automatics have neutral too

Ladogaboy

That depends on what kind of vehicle you have. Some don't have pumps in the fuel tank.

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PrObLy

Well if he did that he would be going very fast down the mountain or destroying his brakes to keep the car at a reasonable speed. Using the engine/transmission to brake won't use any more fuel than simply braking with the brakes because you're not feeding any additional fuel into the engine, just letting the transmission slow up the car with a lower gear.

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johnnymk

First of all, he stated that he was putting the tranny in a lower gear, which naturally increases engine RPM at an equivalent speed. Result: higher fuel usage.

Secondly, going down a hill in the lower gear increases engine vacuum. In this condtion, the engine doesn't really know whether it is accelerating or under a high load. Regardless, the result is higher fuel usage.

If the mountain is steep, the brakes could heat up if coasting in neutral. If I were him, I would coast in neutral with the brakes on, then put it back in gear, and alternate back and forth till I found fuel.

PrObLy

I think I have to disagree about the higher rpm automatically yielding higher fuel usage because using the tranny as a brake doesn't involve the addition of any more fuel to the engine becuase it's the trans that will be naturally spinning at a higher rpm simply because of the speed you are at and the gear ratio, not from the amount of gas you are feeding it since gravity is pulling the car down the hill it is keeping the car near that rpm automatically. I would agree that a higher rpm would yield higher fuel usage, though, if he were accelerating while keeping the car in a lower gear but since his intentions were to slow down, simply downshifting and not hitting the accelerator shouldn't hurt fuel economy moreso than simply braking.

Even if this would by chance waste additional fuel I believe it would be worth running out of gas rather than roasting your brakes. This reminds me of when I was in Palm Springs and while coming down one of the mountains the mini van in front of me was on his brakes the whole time, you could literally smell them burning up for miles.

You would definitely be correct if it was a level street because you would have to use the accelerator to hold the car at a constant ,say, 3000 rpms and use a lot more fuel instead of when going down a hill, letting gravity pull you down and the trans to counteract it.

DaNewbie

I don't know if this applies to other cars, but for the 4th gen honda preludes, the ECU will cut fuel going to the injectors if the car is coasting for a while and detects that the gas pedal isn't being touched. Read about it in the manual

ArkiStan

The differing views seen here accurately reflect the puzzle that went through my head at the time. Do high RPM's necessarily mean the engine is doing work? In the case of engine braking, maybe gravity (or the momentum of the car) is doing the work FOR the engine. In that case:

1. Engine-braking in low gear (3,000-4,000 RPMs)
2. Braking in neutral (idling @ 700-800 RPMs)

Case 1 wouldn't necessarily be consuming any more fuel than case 2. In which case, 1 would be preferable, since I'm saving the brake pads.

Can anybody prove/disprove this theory?

Ladogaboy

Personally, I think that using the transmission for a brake downhill wouldn't consume more gas simply because your foot is off the gas pedal, which controls the fuel pump, which is directly responsible for how much gas is being pumped into the engine. It seems possible for you to be running the engine at 3-4K yet still have the compression ratio in the chambers far lower than normal.

Anyway, what I do know for a fact. My car has over 180K miles, and I compression brake almost all of the time. I am yet to replace the brakes on this vehicle, and I have never noticed a significant fluctuation in gas mileage--even when driving in the mountains. On the other hand, however, my transmission is probably going to have to be replaced in the very near future. Whether, because of compression braking, it would have needed to be replaced sooner or later, I can't tell.

bbrian

Engine breaking is using the compression in the cylinders to slow down the vehicle. Since the throttle is not opened while you are descending down the hill, less air is making it into the cylinders than what would be if you were normally driving at the same RPM'
s, which means less fuel is making into the cylinders as well...

I go rock-crawling with my brother sometimes in his jeep and we have crawled down several rocks in low gear with the engine turned off.

billxp

Assuming the vehicle is a newer fuel injected model. No you wouldn't be using more fuel. The computerized engine management system relies on several sensor, TPS sensor (throttle position sensor), MAP sensor (Manifold Absolute Pressure), Crank Sensor etc.

Going down hill you foot would be off the gas the ecu (car computer) would see this, the manifold vacuum would be high because of the closed throttle plate. Whats really slowing you down is engine compression.

One last thing to remember Brakes are cheaper than engines and transmissions.

Don't need to ride the brakes all the way down just light taps to slow you down to set you up for the fun turns