Comments on: Fuel Economy Tip – Avoid Engine Braking

By: Al

Al — Sun, 26 Jun 2011 03:25:35 +0000

I drive my car on cruise control. When I go down a steep hill, it automatically downshifts to keep the pre-set speed. The RPM increases relative to the steepness of the road.

Question: Which is least expensive? Using brakes or letting the engine do the job??

By: Who wrote this?

Who wrote this? — Sun, 24 Apr 2011 21:06:53 +0000

This is a load of nonsense, as the only time fuel enters the engine is when you push the gas pedal. Rev matching will push some fuel in there to match RPM’s for a down shift but while the engine is being used for breaking you are not touching the gas pedal and throttle is released therefore no fuel is being used. In fact the intake and exhaust valves are closed essentially creating a vacuum effect that slows the car down more than if you were to stay in neutral. Also, engine breaking is essential for optimal engine break-in results. Sure you might use a tad more gas by blipping the rpm’s up for a rev match but that should be negligible.

By: sb

sb — Thu, 10 Mar 2011 02:47:06 +0000

hey ive heard that elsewhere too. so in a modern car, does the computer tell it to give it NO gas, even though it’s in a high rpm? or does it tell it to give only the small default amount per stroke of the engine?

also, you said it burns less fuel when blipping than it does to drop to idle. how does the engine use extra gas when shifting to neutral?

By: Bazoon

Bazoon — Tue, 08 Mar 2011 23:15:56 +0000

Glad to see someone has their head screwed on right.

By: Dan

Dan — Sun, 13 Jun 2010 21:54:02 +0000

Brian Carr, Your article is totally misleading. If not for the good folks on here that have helped straighten the path, I think I may have been fooled into thinking engine braking was a waste of fuel.

Seriously, at least get your facts straight before you mislead people. There is no fun in that. Or change the topic. This is false, one of the reasons people don’t trust the internet. You came on here sounding like a “trusted source” but by all of your arguments and so-called ”logic” it is obvious you know jack-squat about engines and the effect of engine braking. Please stick to your day job (hopefully this is not it).

Thanks

By: Fahim

Fahim — Sun, 31 Jan 2010 23:41:45 +0000

sonyAD, i think i’m in love with you!

Your amazing knowledge of the ins and outs of a modern motor vehicle engine shocks me, and i would love to know, where you have learnt this all from?

I’m currently studying motorsport engineering in college, in my third year of HND level 3, which i want to take to uni.

Could you possibly let me know where you’re studying and how its going etc?

Thanks very much, and once again, amazing!

By: LMF

LMF — Wed, 23 Dec 2009 17:24:17 +0000

Wow… complicated stuff. I kinda feel glad that the future is in electric cars… thankfully the motor is a lot simpler than an engine and also a lot more efficient!

By: SonyAD

SonyAD — Wed, 23 Dec 2009 16:56:52 +0000

LMF, the effect seems to me basically the same between not opening the throttle plate as much and closing the intake valves after compression begins. You still have to pump some of the mixture back out, which expends energy, and you still have throttling losses. That is to say, if ignition were not to occur, the pressure inside the cylinder when the piston reaches bottom dead centre on what should have been the power stroke (assuming the exhaust valves don’t open before BDC on the power stroke) is lower than atmospheric. This means that during the power stroke there is a negative pressure difference between the combustion chamber and the crankcase to overcome in addition to driving the other pistons in their strokes and propelling the vehicle. This is true of any naturally aspirated engine but is even more so for a throttled naturally aspirated engine running under part throttle.

I think what you mean by ‘compression ratio’ is actually the overall pressure ratio. What I mean by ‘compression ratio’ is the ratio between the combustion chamber volume when the piston is at bottom dead centre and when it is at top dead centre.

The pressure ratio is highly dependent on more than just compression ratio, as you point out. It also depends, for example, on atmospheric pressure or pressure inside the intake manifold, if the engine uses forced induction. And a host of other factors.

In my comprehension, one can speak of an engine having distinct and different compression and expansion ratios if that engine’s piston travel during the compression stroke (from piston BDC to TDC) is longer or shorter than the piston travel during its power stroke (from piston TDC to BDC) irrespective of the moment ignition or injection occurs or the intake or exhaust valves open.

Which, I think, is not the case with any automotive engine currently in production, though I may be wrong.

“The goal of the modern Atkinson cycle is to allow the pressure in the combustion chamber at the end of the power stroke to be equal to atmospheric pressure; when this occurs, all the available energy has been obtained from the combustion process.”

This is a bad goal. Nevertheless, if it’s attainable by closing the intake valves during the compression stroke then it’s surely attainable by not opening the throttle plate as much.

As I said before, I think a much more relevant indicator of engine efficiency are specific fuel consumption under part load as well as full load and exhaust gas temperature under various loads and rpm.

By: SonyAD

SonyAD — Wed, 23 Dec 2009 16:17:29 +0000

BTW, have a look at the MCE-5. It’s a (constantly and continuously) variable compression engine which also eliminates piston side thrust. However, there’s also a lot of reciprocating mass. The concept has been in development since ’97 and still has some way to mass production. Apparently the concept has been validated by PSA, which is the first manufacturer licensed to produce such an engine, under the VCRi badge.

Peugeot are one of my favourite manufacturers but I don’t think this design is all that good.

By: LMF

LMF — Wed, 23 Dec 2009 16:03:49 +0000

@SonyAD: Sorry for not explaining the intake valve thing very well. Here’s what I meant (this is copied from Wikipedia): Recently Atkinson cycle has been used to describe a modified Otto cycle engine in which the intake valve is held open longer than normal to allow a reverse flow of intake air into the intake manifold. This is more like a Miller cycle engine than an actual Atkinson cycle engine. The effective compression ratio is reduced (for a time the air is escaping the cylinder freely rather than being compressed) but the expansion ratio is unchanged. This means the compression ratio is smaller than the expansion ratio. Heat gained from burning fuel increases the pressure, thereby forcing the piston to move, expanding the air volume beyond the volume when compression began. The goal of the modern Atkinson cycle is to allow the pressure in the combustion chamber at the end of the power stroke to be equal to atmospheric pressure; when this occurs, all the available energy has been obtained from the combustion process. For any given portion of air, the greater expansion ratio allows more energy to be converted from heat to useful mechanical energy meaning the engine is more efficient.

The disadvantage of the four-stroke Atkinson-cycle engine versus the more common Otto-cycle engine is reduced power density. Because a smaller portion of the compression stroke is devoted to compressing the intake air, an Atkinson-cycle engine does not take in as much air as would a similarly designed and sized Otto-cycle engine.

Four-stroke engines of this type with this same type of intake valve motion but with a supercharger to make up for the loss of power density are known as Miller cycle engines.

If you were to close the throttle plate, wouldn’t that increase pumping losses and reduce overall efficiency?