|Forum | Marketplace | Knowledge Base | | H1 site | H2 site | H3 site
The Hummer Knowledge Base
>I don't understand why gas would have better engine braking than diesel. In fact, in my experience engine braking is a function of the engine resistance to rotation, ie: compression ratio. In my experience over the years, gas engines with higher compression offered more braking than lower compression (emission engines from the mid '70s through '80s). Since a diesel compression ratio is roughly 22:1 as opposed to a gas engine @ around 9:1, it seems to me, both in driving experience and logic, that the diesel brakes better than the gas.
I thought the same thing when I first heard it. I wrote the editor of the GM Diesel page and this is what he wrote back:
A gas engine has more engine braking than a comparable displacement diesel because at low throttle levels a gas engine is working against a closed throttle plate. A diesel has a wide open intake manifold without a throttle plate.
Think of a diesel piston and cylinder like an air cylinder. If you press the piston up to near TDC with the valves closed, it takes a lot of pressure, right? But guess what, after rotating beyond TDC all that compressed air now pushes down on the piston to accelerate it to BDC.
A gas engine will generate about 25" of vacuum with the throttle closed, so when the piston comes up to TDC, very little pressure is there to push the piston on the down stroke. Then, when it wants to pull in a fresh charge, the engine has to pull against the vacuum. All this consumes energy and creates what is termed "compression braking".
Of course, a diesel does produce some compression braking due to mechanical losses and heat generation, but a similar displacement gas engine will always generate more "compression" braking.
I hope that makes it clear.
Jim Bigley, Editor
>On another thread on engine braking. Yes, gas engines can produce a higher braking hp than a diesel. However, like "normal" hp, it is a function of rpm. You need to get up in the rpms to get the maximal braking. However, under wheeling conditions, you usually don't need maximum braking like if you were towing. Instead, you want more braking hp at a lower speed which a diesel does provide. The "braking torque" curve is flatter.
Gerald, the only thing that's braking the diesel truck is the drive train friction. As you well know, when you are driving off road in low lock, low gear, descending a steep hill, the engine and drivetrain is forced to turn at a higher RPM than when driving in say high gear. Because the engine and drivetrain are turning at a high RPM they produce a lot more friction, thus more stopping, than in the high gear. This stopping power is due to drivetrain and engine friction, not engine braking in the sense of a gas motor.
Engine braking is achieved when the pistons have to overcome the high vacuum created in the intake or push against a restriction in the exhaust in order to fall. Because a gas engine has a plate that blocks the entry of air into the intake manifold when the engine is decelarating, the pistons will have to fight the vacuum created in the intake in order to rise and fall. A diesel has a wide open intake and controls its throttle by varying the amount of fuel the injectors pump into the cylinders.
There are engine brakes available for diesels. In fact, they are common on the big rigs. What they are is a valve that restricts the exhaust. When the engine is throttling down a valve chokes off some of the exhaust flow. This means the pistons have to overcome the compression in the exhaust manifold in order to rise.
The reason that your experiment with the car doesn't apply to the Hummer is that the Hummer is so heavy. As I wrote on the list before, when I have my 7000 pound diesel truck on the highway going 65 and I go to exit I have to use the brake or downshift to slow the truck down because it just keeps on gliding very fast for quite a distance. When I had my gas truck and did the same it would slow down from 65 to 45 almost as soon as I took my foot off the gas. Instead of using the brake I found myself using a little throttle once I got part way down the ramp. Off the road when in low lock low descending a steep grade I found that the diesel would walk down faster causing me to use a little brake. When going over the edge in a gas truck it would sorta hang and walk down very slowly with no brake needed. On the other hand the diesel would just walk up the same hill at a ridiculously low rpm near 1500. This torque at low RPM is very strange for someone that has always had gas trucks. Because the diesel so casually rolled over the terrain it got fantastic fuel milage. Better than double the gas truck.
Last year I had my gas truck in Moab in August and wondered all over alone with no problem. This year I had the diesel truck in Moab in August with no problem.
> Gerald, the only thing thats braking the diesel truck is the drive train
Since we all have been dying for some technical content, let me put in a few more cents if you don't mind :). I do not disagree that the drivetrain friction has something to do with the overall braking. In fact, as you say, friction is basically how it works. As is pointed out else where, "compression" braking is a misnomer and "engine" braking is more correct. So I was using engine braking to mean powertrain braking. However, I am going to still say engine braking b/c given a gas and diesel Hummer, the drivetrain is the same, so differences are from engines only :). How's that for a justification!
What is a little more technical is that as the air is compressed in a diesel, it heats up and this heat does not all go out the exhaust, i.e. it is not adiabatic. Point being that the higher compression does add some significance to the equation.
> Engine braking is achieved when the pistons have to overcome the high
Well, I have to nitpick. Engine braking is all braking effects from the engine. Basically any drag is a braking effect. I agree that your explanation of the intake restriction and its effects. However, there are many other effects. First, a gas motor is still burning fuel. The amount of fuel and air in the cylinder is still close to ideal. Point being that it still produces power. The engine does breath quite a bit still. Sure, a diesel uses some fuel too but it is extremely small b/c the inj. pump goes to idle settings off throttle. Another point is that the compression of the air on the up stroke and expansion (air spring) is not adiabatic and is a function of compression ratio. Hence, higher compression does add a significant braking effect. Remember we are almost talking 3x here. Diesels take quite a bit of energy to keep spinning.
> by varying the amount of fuel the injectors pump into the cylinders. There
You are talking about an exhaust brake. Big rigs more commonly use a Jake which is quite different. It does not block the intake or exhaust, instead venting the cyl pressure at certain times so that the pistons pull against a vacuum like a gas engine. However, your point is very valid about adding a braking device. Oh, getting back to the "engine" braking def above...many of these companies have run tests of engine braking w/ and w/o an external brake. The amount of natural engine braking (no powertrain) is quite significant.
> The reason that your experiment with the car doesn't apply to the Hummer is
Interestingly my experience has been quite the opposite. I don't doubt your numbers at all. Do you have a turbo diesel by chance? They seem to be really poor on "engine" braking - much, much different than the NA diesels. Just trying to get a data point...
I hope we haven't bored anybody to death.
Back to main page