Bogus explanation in "His tombstone can read, "He got 42 miles...""


#1

Yet again, the Brothers claim that engine braking (putting the car in a low gear slows the car down when coasting) is that the engine’s mass absorbs the momentum like a flywheel. That isn’t the case, because if it were, that stored energy would help the car coast up over the next hill.



Engine braking works by friction, just like the brakes do, except the friction here is between the moving parts in the engine. That friction helps make the engine hot. That heat goes into the coolant, then through the radiator, and out into the air. I.e., the energy isn’t stored, it is just lost.


#2

Yes, they were wrong, but you aren’t exactly right either. Engine braking comes from compression. The braking energy is dissipated by compressing air and blowing it out the exhaust pipe(s).


#3
...they were wrong, but you aren't exactly right either. Engine braking comes from compression...
I don't think you're correct there. Engine braking does come from compression - in a diesel engine with a compression release engine brake (Jake Brake). However, downshifting in a car uses the engine vacuum caused by closing the throttle, along with the friction in the drivetrain (like they said) to help slow the engine. It's not engine compression being dissipated through the exhaust unless he's driving a semi truck with an engine braking system!

#4

I too come down on the side of compression. Of course I’m often wrong.

There’s an easy way to test this: Car in gear, engine off, foot off the brake. Let it roll down a hill of known distance and time it.

Same scenario, but take out all the spark plugs.

Which way makes the car go slower? And by how much?

If no hill is available, try the same two trials, but just turn the radiator fan or push the car.


#5

Take out the spark plugs and you eliminate the effect of the vacuum created by closing off the flow of air into the engine, so of course it’s not going to be slowed as much. Compression braking (like is done on semi trucks) is a whole different scenario. The reason trucks are so loud when the Jake Brake is activated is that the braking system opens the exhaust valves after the piston compresses air in the cylinder - that’s not happening in a gasoline engine. And if the engine weren’t releasing that compressed air, it would just be working to push the piston back down once the crankshaft turned over so that the piston was on a down stroke.


#6

Engine braking by air compression, AFAIK, doesn’t happen in passenger cars with gasoline engines. That would require the exhaust valves to open quite early in the combustion cycle. And anyway, the engine is idling, so combustion is still providing enough power to push the engine around. It just doesn’t provide enough power to drive the engine at the speed that the slope and given gear would require. Hence the braking effect.


#7

Ah, you are saying what I meant to say. But the Bros didn’t actually say friction in the drivetrain slows down the car. They talked about the engine’s “mass”, whatever that means. And they have said previously that the engine stores momentum like a flywheel, i.e., the inertia in the engine’s spinning parts provides resistance to acceleration. So to be precise, they didn’t claim some sort of flywheel/inertial effect explicitly, it seems safe to assume that is what they meant.


#8

You’ve all missed the point. The husband-driver was trying to enhance his fuel mileage by coasting down the 2500 feet from work to home, and he did so by putting the car in neutral, thus foregoing engine braking and perhaps some safety. The brothers favored leaving the car in gear on safety grounds, noting that the modest fuel savings were eclipsed by the safety issue.

Bogus, cuz they should have noted that the car (a 2001 EFI Sentra) will use less fuel if it is left in gear as he coasts down the mountain, than if it were shifted into neutral; hence shifting the car into neutral didn’t enhance fuel economy – it lessened fuel economy.

That’s because the ECM on the Sentra – as with all modern EFI cars – cuts off ALL fuel to the injectors during sustained periods of high manifold pressure. In contrast, when the engine is idling out of gear, the ECM permits enough fuel to maintain the idle.

Had the driver left the car in gear all the way down the mountain, he would have had the advantages of engine braking AND he would have enjoyed better fuel economy. Not much better, but since fuel economy was his quest, the brothers rendered up BOGUS advice.


#9

Halmca is sort of on to something, but I just don’t get this manifold pressure deal. I do understand the fuel cut operation. On my old Honda, the ecm will, once the engine has reached proper temperature, cut fuel on deceleration. When the ecm senses that both the throtle is closed and that the engine speed is above about 1200 rpm, it cuts the signal to the fuel injectors. On some grades you can feel the car slightly surge forward (engine braking is slightly lessened), as the engine speed drops through that 1200 rpm threshold. Of course if the grade is steep enough the rpm will remain above 1200. Then, the fuel will remain cut off until the engine temperature drops to its threshold level. I don’t know exactly what the temperature might be, but the system works well in my Honda.