Fuel Economy Tip - Avoid Engine Braking
Today's tip is yet another easy way to save money and wear and tear on your transmission.
While down shifting is an easy way to take it easy on your brakes, it's also a great way to needlessly waste gas.
Anytime your engine revs and the RPMs increase (which is what happens when you down shift), you will use more gas than you would at the same speed but in a higher gear (lower RPMs).
So, instead of using your engine to brake go ahead and ease on to your brake pedal and smoothly come to a stop.
Avoid engine breaking.
Essentially, engine braking is when you down shift the transmission in order to slow down your vehicle. For example, you're going down a road in fifth gear, you see a red light ahead and instead of slowing your car by using the breaks, you down shift into third gear.While down shifting is an easy way to take it easy on your brakes, it's also a great way to needlessly waste gas.
Anytime your engine revs and the RPMs increase (which is what happens when you down shift), you will use more gas than you would at the same speed but in a higher gear (lower RPMs).
So, instead of using your engine to brake go ahead and ease on to your brake pedal and smoothly come to a stop.
posted by Brian Carr at 7:57 PM
12 Comments:
Nonsense.
Engine management can detect the situation and cut off the injectors for the duration.
It doesn’t work as you describe.
By “down shifting” you have basically changed your engine into a compressor.
Now as the car rolls forward you’ve set it up, by downshifting, so that the flywheels continue around and CONTRIBUTE energy to the compression stage (and all the others) instead of removing energy to move the car against all the frictional forces.
Since less Work is required to cycle around to the next compression stage, and modern cars sense this, less gas is added in the compression phase.
Real world example: You (engine) are pushing a person on a swing set. To overcome the frictional forces you HAVE to keep pushing by a certain amount. Now the person starts pumping their legs to help out; you have to push LESS (use less gas) to maintain the same height of the swinger.
Also the higher RPMs are casued by the car's momentum helping the compression stage not additional fuel expenditure.
So either way you are spending money. Even if it worked as you said with down shifting wasting more gas, you'd be using your brakes more, which in return, would mean you would be replacing your brakes more often. Brakes cost money too, especially if you aren't mechanical enough to replace them yourself, you'll be paying more for a mechanic to do so. All in all, in the end you spend money everytime you jump into a vehicle in some fashion. That's just part of owning a vehicle. :)
True, but over time brakes cost a heck of a lot less than what it costs you to fill up on gas. You might have to replace your brakes every 25,000 miles (give or take) but you have to fill up your gas tank every 350 miles.
And engine braking doesn't use MORE gas.
So you mean to tell me that 50 mph in 5th gear at 2,500 RPMs uses the same amount of gas as 50 mph in 4th gear at 3,500 rpm?
Normally, no you are right more rpm means more gas.
BUT IN THIS CASE, YES.
The "extra rpm"s come from "the road". You are using that car's momentum to get those extra rpm NOT more gas.
It seems to me these things aren't mutually exclusive - it doesn't matter how you get to higher RPMs, just that you did get to higher RPMs. Higher RPMs lead to increased fuel consumption, thus the logic.
##“It seems to me these things aren't mutually exclusive - it doesn't matter how you get to higher RPMs, just that you did get to higher RPMs.”
YES, it does matter “where it comes” from that’s the whole point. What you have to ask yourself “Is where are the extra RPMs coming from?”
##“Higher RPMs lead to increased fuel consumption, thus the logic.
Your logic is incorrect; you are starting from a conclusion and working backwards. That isn’t how logic works.
Look, I've explained it as best I could. Google "engine braking" for heaven’s sake. Wiki and answerbag.com both say the same thing. Ask an engineer in RL because face-to-face makes it easier to explain something.
Say you have to push to jumpstart your car. What is happening? You push the car to get it going. When you pop the clutch the momentum of the car is transmitted to the pistons causing them to rise and fall (hopeful through the compression cycle and trip the spark plug). This isn’t using more gas and neither does engine braking.
Here is another similar example. In a hybrid, regenerative braking recharges the batteries, right? How does this work? It turns the kinetic energy of moving down the road back into electrical energy that is then restored to the battery, effect car slows down, and the battery is recharged. (According your “logic” regenerative braking would use MORE energy from the battery.)
Engine braking TAKES the energy of moving down the road and changes it to moving the pistons “up and down”. To put it very simply, it’s the energy it takes to move those pistons that works to slow the car down. Now instead of “making more gas” (as in the hybrid example) this work is lost to the engine as heat but it DOESN’T USE MORE GAS TO INCEASE THE RPMS caused by engine braking.
-Normal driving say is 2000 RPM. All from gas.
You accelerate (depress gas) 4000 RPM all from gas. (yes, this uses more gas than normal.)
- Engine braking: 4000 RPM. That’s 1000RPM from gas, 3000RPM from the car slowing.(net effect LESS GAS.) It’s less rather than the normal above because the computer in the car knows how much gas it needs and with engine braking it is senses it needs less.
Park on a steep slope.
Put car in first.
Push car.
That's RPMs without using gas.
Engine braking is the same.
I was just having the same argument with my mate. I did a search to try to prove him wrong and I found this. I was arguing that It would use the same amount of petrol (gas - im from aus) when you gear down. The argument we were having was identical to the one here :)
I think that even in a carburated car it would not use more gas because the engine isnt given any more gas by the carby because it dosnt need it. As Penty described - its the kinetic energy that is truning the engine at higher than normal revs.
Think about this; If the extra rpm was being produced using extra gas, where is the kinetic energy from the momentum of the car going? I think you explained it well penty, but its hard to explain...im having a very frustrating argument with my mate.
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