Fuel Economy Tip - Avoid Engine Braking
June 1st, 2006 | by Brian Carr |Today’s tip is yet another easy way to save money and wear and tear on your transmission.
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.
31 Responses to “Fuel Economy Tip - Avoid Engine Braking”
By Anonymous on Jun 1, 2006 | Reply
Nonsense.
Engine management can detect the situation and cut off the injectors for the duration.
By penty on Jun 1, 2006 | Reply
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.
By penty on Jun 1, 2006 | Reply
Also the higher RPMs are casued by the car’s momentum helping the compression stage not additional fuel expenditure.
By Anonymous on Jun 2, 2006 | Reply
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.
By Brian Carr on Jun 2, 2006 | Reply
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.
By penty on Jun 2, 2006 | Reply
And engine braking doesn’t use MORE gas.
By Brian Carr on Jun 3, 2006 | Reply
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?
By penty on Jun 5, 2006 | Reply
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.
By Brian Carr on Jun 7, 2006 | Reply
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.
By penty on Jun 8, 2006 | Reply
##“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.
By penty on Jun 8, 2006 | Reply
Park on a steep slope.
Put car in first.
Push car.
That’s RPMs without using gas.
Engine braking is the same.
By g prime on Oct 7, 2006 | Reply
First off, penty, you are 100% right on the subject.
Brian Carr… Don’t post if you don’t know what you are talking about. Even someone that knows absolutely nothing about engines can use the conservation of energy law to figure out that if a car isn’t being propelled forward, that it is not using any energy. Think about it. The engine’s computer has a sensor that detects throttle position. If the throttle is closed, then the driver clearly doesn’t want any power going to the wheels. The engine cuts off the fuel and the engine starts acting as a compressor. Thats why the car slows down. The engine is taking the energy from the vehicle in motion, and using it to compress air. If the computer dumped in more fuel then the car would accelerate. The energy would have to go somewhere. The lack of acceleration proves the absence of energy and therefore the absence of fuel. You were correct however that under normal driving situations, more rpm means more fuel, but there are exceptions to that statement.
By Det ExPat on Mar 20, 2007 | Reply
I was searching the web about this topic and was surprised to find this “Fuel Economy Tip”. I do not know whether or not this blanket statement is true for every vehicle or every powertrain, but I have been experimenting with engine breaking on my Saab 9-2x aero for about a month and have found in simple milage calculations (trip computer miles / gallons per fill up) that I have increased my fuel economy by at least 2 mpg by down shifting to brake my car at stop signs on my daily commute. I know this is a sample size of 1 anecdote, but I was wondering if anyone here knows whether this is typical in modern vehicles. I know that my engine is a turbocharged boxer 4 cylinder (the car is a rebadged Subaru WRX after all) and was wondering if the advanced Subie engine or turbo help contribute to the mpg bump. Should I be worried about wear and tear on the engine? Any advice will help.
That said, are there certain engines (brands, configs - eg I4, I6, V6, V8, or technology options - e.g. DOHC vs. OHV?) that get a better boost (or severly worse as the comment suggests) in mpg from engine braking?
By the way, if Penty is corrrect and engine breaking really boosts engine efficiency, shame on you dailyfueleconomytip.com - take down this message -(after the wise posters help answer my questions of course).
By Brian Carr on Mar 21, 2007 | Reply
Det ExPat - thanks for the comment. It’s always been my experience that I get worse gas mileage the more I downshift / use engine braking. That being said, like all of these tips, this tip isn’t meant to be the be-all-end-all. It’s not going to be applicable to every make and model, so you need to figure out what’s best for your particular vehicle.
By no one on May 2, 2007 | Reply
brake checking however not a gas saving tip is a sweet way to mess up someone elses car and maybe even urs and not have to pay a dime to fix it since some ass hole rides ur bumper. brake checking is only advisble in 1/2 ton trucks or above brake checking a smaller car or even a little truck like a ford ranger do not try to take on a semi u will die for sure.
By Anonymous on Sep 2, 2007 | Reply
Thanks for posting Penty. Good explanation. Brian, you can learn a few things here.
By LMF on Dec 3, 2007 | Reply
A fuel injected car will indeed probably shut off fuel during engine braking, but I’m not so sure a carburetted car can do the same. Probably, in a carburetted car, some extra air is sucked through the air intake (even with throttle valve closed) and unfortunately, this air might suck some extra fuel from the venturi too. So you really have to grab a carburetted car with a fuel consumption gauge and drive it down the same hill both when idling and when engine braking. And while you’re at it, do the same for a fuel injected car and tell us what you find.
P.S., engine braking DOES exert more wear-and-tear on the engine (if it idles at 1000RPM but goes 2000RPM under the effect of engine braking, you have effectively doubled the rate of wear and tear because the pistons now move twice the distance in the same amount of time). So the question now becomes: will the wear and tear be significant? Will it outweigh the cost of replacing the brakes prematurely?? Somebody should test this too…
By John on Dec 4, 2007 | Reply
Engine braking is not necessarily more wear and tear on your car, it’s good to reverse the forces normally acting on the pistons sometimes. Instead of pulling, they are pushing on the crankshaft. The heads are being pushed out and pulled in as opposed to pushed. This is good for compression rings as well. Always rev-match though, don’t want to wear those synchronizer gears too much!
By John on Dec 4, 2007 | Reply
Det ExPat - I engine brake on my STi all the time, she loves it. I log every fillup, I have not noticed any spike in fuel usage downshifting or not. Also I think Saab threw some softer (more comfortable) suspension on your version before they swapped badges
Sweet car though.
By whammy on Dec 4, 2007 | Reply
The idle circuit in a carburettor will indeed continue to supply fuel to the engine if the vehicle is coasting, in fact it can supply more fuel than is required due to the excessively high intake manifold vacuum. The other primary circuits in the carb will not be in effect (acceleration, mid and high range enrichment circuits).
Some other bad things about excessive intake manifold vacuum: reversed loads on the connecting rods can cause the caps to ‘pinch’ and the bearings to fail or wear abnormally, piston rings experience artificial increases in tension which causes abnormal wear or breakage, oil seals in the upper valvetrain weep more oil causing combustion chamber and valve stem contamination and coking.
The short answer is: coast when you can, brake when you must, engine brake when your regular brakes fail.
Still think it’s cheaper to engine brake? OK, Riddle me this, Batman: Which is cheaper, a brake job, or an engine rebuild?
By LMF on Dec 5, 2007 | Reply
Dear John:
Allow me to clarify what I mean by “more wear and tear”.
As you know, the pistons in an engine move up and down within the block. The o-rings (or whatever they’re called) are the things between the piston and the wall of the cylinder. Now, while the engine is turning, the pistons are moving up and down and therefore are sliding within the wall of the cylinder. There is a very slight amount of friction between the sides of the piston and the sides of the cylinder (as you’ll see if you try to turn the flywheel of an engine with its spark plugs removed - there’s quite a bit of resistance, and it’s not due to compression because you’ve removed the spark plugs, so it must be due to piston and bearing friction).
Now, if you’re coasting down the hill with the engine doing 1000RPM, in 1 minute the piston will have slid up & down a distance equal to 1000 x 2 x stroke distance (the 2 is there because each rev gives BOTH an upwards movement an a downwards movement to each piston). Now, if you engine brake and the engine goes 2000rpm down the same hill, in 1 minute the pistons move 2000 x 2 x stroke distance. So clearly in the second situation your pistons have traveled twice the distance (twice the rubbing), and therefore you’ve given them twice as much wear, in theory.
In practice, it will probably equate to more than twice as much wear as there is not a linear relationship between velocity and rate of wear…
Anyways, look at post #20. Whammy mentions good reasons that in my opinion might put an end to this argument (however, I’d like to know whether the engine or the brakes would fail first, in a car that uses engine braking and a car that only uses brake pads respectively: in other words, maybe the engine used for engine braking lasts 200000miles, whereas should you use only the brakes, your brake pads might last 100000miles or 300000miles… which is the right answer?)
By M on Apr 3, 2008 | Reply
oh, what a pile of crap. a nonsense and misguiding tip.
By Sumin on Apr 19, 2008 | Reply
I was climbing a mountain with my car, and the fuel gauge needle had significantly dropped after 30miles of climbing.
When I was going down, I was using engine brake all the way down. The RPM was around 2000-3000 with 3rd and 4th gear. The needle had barely dropped (almost unnoticeable)
That leads me to believe engine braking is a good way to save fuel.
By Duh on Apr 22, 2008 | Reply
Sumin - use your brain. Of course you’re going to use more gas going up a mountain than you would going down a mountain. You think gravity might have something to do with that one?
By Eric on Apr 29, 2008 | Reply
Wow, I was checking out the thread and I can’t believe that people are actually arguing over a common-sense concept! Engine braking is transferring the car’s kinetic momentum energy to the energy needed to overcome the engine’s compression cycle. Since the energy is moving in the opposite direction.. from the transmission to the flywheel, to the crankshaft, to the connecting rods, to the pistions, etc… there is no need for an “explosion” from the piston to overcome the next compression cycle.. the energy is already there (car in motion). To make a general statement that more RPM’s means more gas consumed is really looking at this situation blinded. You can actually cut the gas line to the engine and will continue to run at those high RPM’s as long as the car is still moving downhill. Once the car reaches flat ground it will come to a stop since the energy needed to overcome the engine’s compression will no longer be there.
Maybe the confusion with thinking RPM’s with a moving engine = gas consumption is coming from the ancient days of carburetors which are still used today on our gas powered lawnmowers. The concept there is that the gas is drawn into the cylinder by the “venturi effect” where the air intake for the engine passes over the gas line and creates a gas line suction (kind of like blowing over the top of a soda straw you can see the liquid rise up the straw). So, in this kind of engine.. YES, fuel is consumed whether the engine is actually running by combustion or by being a brake since in both cases air is still being drawn into the engine forcing gas to flow in as well.
THIS IS NOT THE CASE with fuel injection engines. With the ignition turned off ie (fuel pump and injectors not running) the only thing that is entering the cylinder if the engine is in motion from braking the car is air itself.. no fuel at all.
I hope this sheds complete unfiltered light on the topic at hand
By Bob on Jun 9, 2008 | Reply
Google hypermiling for lots of interesting info on how to save gas.
Common sense is only right when it’s right. In the case of this argument, the fact that modern cars use injection and computing to regulate gas flow is key. Because of this, the engine does not suck any gas through when coasting. It uses some gas (about 0.4 gallons per hour in a V-8, 0.6 with air conditioning on) when idling, however.
By Eugene on Jun 22, 2008 | Reply
If engine braking used that much more fuel. There would be alot of major trucking companies telling their drivers not to gear down just to use their brakes. Engine braking only slows you down about 5 mph when you drop a gear. When driving a 40 ton vehichle you can’t rely on brakes to stop you. You have to engine brake no matter how high your rmps are going to shoot up. And it takes about $1,000 to fill up a tractor trailer so you do the math. Engine braking does not waste that much more fuel.
By screw driver on Jun 25, 2008 | Reply
It does cause _me_ to use more gas. I’m traveling at high speed with a long straight road in front of me so I want to coast down the road. I take my foot off the pedal and notice the car slows down to 30KM really damn fast. By putting the car in neutral before I release the gas pedal it avoids engine braking and the car actually coasts freely for a good stretch of road
By mankku on Jul 9, 2008 | Reply
Somewhere people were discussing whether brakes or the engine will need replacement first if engine braking. I would dare a guess that regular brakes need replacement first, basing my guess on the fact that engines may run for two or three hundred thousand kilometres at varying RPMs.
I drive a decent amount in the countryside and with my Corolla in 5th gear, the engine usually does around 3000 RPM at around 80 km/h. At 100 or 120, the RPMs increase to close to 4000. Assuming that the average RPM is 2000-2500, engine braking with that same average would seem to me like normal wear and tear.
I have used engine breaking ever since I got my license, which is almost 4 yrs. For my car, engine braking works when the RPMs are above 1500. When the car slows down enough to bring the RPMs lower than that, the system starts injecting fuel and the zero-consumption stops; thus, gearing down a bit more raises the RPMs above the limit and cuts consumption. This is basically the idea of engine braking.
You are driving along the road, notice a red light some distance ahead and gradually gear down as required, leaving the clutch up in between gearing down. The motor will spin at some RPMs but will not consume any fuel as long as RPMs stay sufficiently high. Gradually gearing down as required will allow engine breaking for considerable distances with zero consumption.
Additionally, the driver learns to pay attention, plan ahead and check for clues of what to do.
By Anonymous on Jul 11, 2008 | Reply
From wikipedia:”By dramatically increasing engine RPM, engine braking causes additional fuel consumption, even through a fully released throttle, in cars without overrun fuel shutoffs.”
So: no “overrun fuel shutoff”- no better gas mileage, yes “overrun fuel shutoff”- yes better gas mileage. Too simple?
I would think that most modern cars(any car built after 85 or so) would have a computer that would down-regulate fuel if under an engine breaking situation. To see mileage benefits, it seems there would have to be some intelligent sensor that could alter the operation of the engine.
The situation would be different from car to car, but most modern cars should be fine in the fuel department.
I wonder how automatic and manual transmissions compare in this respect?
By Anonymous on Aug 24, 2008 | Reply
*** “Still think it’s cheaper to engine brake? OK, Riddle me this, Batman: Which is cheaper, a brake job, or an engine rebuild?”
Stupid question. Engines are meant to take the abuse of years and years of varying RPM situations. They are BUILT that way. Brakes are designed to wear down, converting your kinetic energy of motion into thermal energy and brake dust. OF COURSE a $300 brake job is less than a $500-1000 rebuild, not to mention the downtime.
But that’s not the point. Engine braking saves fuel primarily in fuel-injected cars. Think TPS for a minute. Under accelleration, the pedal is down, the TPS (Throttle Position Sensor) picks up on this. It sends the signal to the ECM which, further downstream, causes the fuel rail to light up and inject gas in an amount directly proportional to the amount of throttle you are giving it. Under engine braking conditions, the throttle isn’t depressed, so the TPS senses this and maintains a 0% throttle, effectively turning the engine into an air compressor.
As for the persons comment about more wear:
*** “…reversed loads on the connecting rods can cause the caps to ‘pinch’…”
The connecting rods aren’t made of tin, they are very rigid and are machined to very tight tolerances, fitting together better than anything you can imagine. The reason that they keep from pinching is because of an elasto-hydrodynamic oil film, maybe only 1 micron thick, keeping the actual surfaces from ever really touching the journal in the crank. And these engine designers weren’t born yesterday: They have a plethora of GENERATIONS of people behind them that all have contributed to the design of the internal combustion engine.
*** “…bearings to fail or wear abnormally, piston rings experience artificial increases in tension which causes abnormal wear or breakage, oil seals in the upper valvetrain weep more oil causing combustion chamber and valve stem contamination and coking.”
That is a possibility, but think of the forces acting on the bearings and rings. They are recieving the EXACT same force during normal operation, just opposite. The axis of force is not changing, therefore the bearings will attain wear no more abnormal than under standard conditions, unless you are implying that the bearings are only meant to have force on them in one certain spot, in one certain way, making the total bearing only about %50 useful. You might be on to something about the rings, but again, they are not made of some sub-grade alloy, and are subjected to the same frictional wear as in normal operation.
-DH