It was mentioned thread about the new Diesel Colorado that it has engine braking.
Since since EcoBoosts and other GDI engines are blurring the line between gas and Diesel technology, could a GDI engine have engine braking like a Diesel? Would it require additional hardware, or are they at the point that software alone could do it?
HappyAndy wrote: It was mentioned thread about the new Diesel Colorado that it has engine braking. Since since EcoBoosts and other GDI engines are blurring the line between gas and Diesel technology, could a GDI engine have engine braking like a Diesel? Would it require additional hardware, or are they at the point that software alone could do it?
My 2011 F-150 Ecoboost had it, sort of. In Tow/Haul mode going downhill, it would downshift and use the engine to keep the speed down. Not a true exhaust brake like a diesel, but something.
All gas engines have the ability to engine brake, even the most primitive carb-ed ones. There's significant drag generated by spinning the engine at high rpm with a closed throttle (it takes energy to attempt to pull air past the closed throttle plate). Fuel injection does make engine braking more effective due to decel fuel cutoff and the ability to close down the idle air flow when engine braking. But even without this, the braking effect is still significant once you get the revs up enough (especially on large displacement engines).
Diesels on the other hand have no significant natural engine braking due to the lack of a throttle, which is why some use exhaust brakes or compression braking devices to generate braking force.
rslifkin wrote: Diesels on the other hand have no significant natural engine braking due to the lack of a throttle, which is why some use exhaust brakes or compression braking devices to generate braking force.
And JUST for braking, there's no reason to put hardware on the engine- way too much money for the gain.
BTW, GDI and diesel are still really, really far apart.
I'm pretty sure my 2012 ford focus would downshift on hills when using cruise control to keep you from 'overspeeding' too.
That said, I have always been mixed about engine braking altogether. My thought is that braking is always taking rotational energy and turning it into heat through friction, and for the same amount of braking I am going to create a similar amount of heat, so, if that is the case, wouldn't I rather put that friction/heat load on my brakes which are easy and cheap to replace rather than put that friction/heat load on my engine internals which are not?
alfadriver wrote: And JUST for braking, there's no reason to put hardware on the engine- way too much money for the gain.
I dunno if I'd say that. For maintaining speed down a steep hill with a heavy load, engine braking can be very, very useful as it has less heat related concerns than the service brakes do in continuous use. And it doesn't generally carry the same rotating and unsprung weight penalty you get from up-sizing the rotors, etc. on the service brakes for more heat capacity.
rslifkin wrote:alfadriver wrote: And JUST for braking, there's no reason to put hardware on the engine- way too much money for the gain.I dunno if I'd say that. For maintaining speed down a steep hill with a heavy load, engine braking can be very, very useful as it has less heat related concerns than the service brakes do in continuous use. And it doesn't generally carry the same rotating and unsprung weight penalty you get from up-sizing the rotors, etc. on the service brakes for more heat capacity.
The throttle allows enough engine braking to not need valve deactivation. Which costs a fortune.
edit- given the fact that the GM diesel in question has certain hardware, I'd bet that the engine braking for the pick up is a throttle, and not a jake brake as is seen on tractor trucks.
Robbie wrote: I'm pretty sure my 2012 ford focus would downshift on hills when using cruise control to keep you from 'overspeeding' too. That said, I have always been mixed about engine braking altogether. My thought is that braking is always taking rotational energy and turning it into heat through friction, and for the same amount of braking I am going to create a similar amount of heat, so, if that is the case, wouldn't I rather put that friction/heat load on my brakes which are easy and cheap to replace rather than put that friction/heat load on my engine internals which are not?
Well, under deceleration with the fuel off, the energy that the engine absorbs goes straight into the exhaust stream. And all of that heat is MUCH less than the heat of combustion.
So engine braking doesn't wear anything out nor heat anything up.
(as an aside- I really enjoy a well calibrated "hill mode"- drives really nicely)
alfadriver wrote: I'd bet that the engine braking for the pick up is a throttle, and not a jake brake as is seen on tractor trucks.
Most likely an exhaust brake, so yeah, basically a throttle plate in the exhaust. Jake brakes are more effective, but also more costly and complex (hence why they're rarely seen on smaller diesels).
In reply to alfadriver:
So the GM small Diesel has a throttle valve? What job would it be doing other than engine braking?
As I said, lines are getting blurred, gas engines without throttles, and now Diesel's with them.
In reply to HappyAndy:
EGR. The throttle will draw enough vacuum to bring EGR in at a rate that it will be effective. Which is a better solution (IMHO) than restricting the exhaust flow by increasing the back pressure for it to flow. So I bet it's on the intake. Not to say that throttles in the exhaust won't happen.... Just that it's cheaper in the intake.
Really, I should not call the plate a throttle on a diesel- the word throttle means you control the output- on gas engines it's done by controlling the air flow, on diesels, it's done by controlling the fuel flow.
But it's hard to call it something else, since it's the exact same hardware that gas engines use as throttles.
Vacuum control butterfly valve. VCBV.. 
The Colorado does not have a compression relief brake AKA "Jake Brake", like big trucks, which actually open an exhaust valve at TDC. Instead it has an "exhaust brake", which means it just closes the turbine inlet (VG turbo) causing exhaust pressure to build up and brake the engine. Cummins and Ford do the same thing with their pickup engines. VG turbos are cool.
Intake throttles on diesel engines are becoming common due to emissions. It's a good way to suck extra EGR down the hatch to control combustion temps. Sometimes known as an "air management valve". It's not really for braking. The braking effect of VG turbine throttling is way higher.
alfadriver wrote: In reply to HappyAndy: EGR. The throttle will draw enough vacuum to bring EGR in at a rate that it will be effective. Which is a better solution (IMHO) than restricting the exhaust flow by increasing the back pressure for it to flow. So I bet it's on the intake. Not to say that throttles in the exhaust won't happen.... Just that it's cheaper in the intake.
VW puts throttle bodies in the exhaust to force EGR instead of in the intake, now.
Yep.
In reply to Knurled:
Yea, I don't really put much into VW tech anymore...
But I do see the point.
IIRC, the 7.3 powerstroke / T444E has a throttle in the exhaust. Mostly used to create extra backpressure for faster warmup.
alfadriver wrote: In reply to Knurled: Yea, I don't really put much into VW tech anymore...
Mind you, it's a rather expensive part, and being a moving component with electronics attached (it looks exactly like a DBW throttle body in layout) in the EXHAUST SYSTEM it is about as failure prone as you would expect.
Oh well. If they're rich enough to buy an expensive Diesel car, they can afford to dump $1500-2000 on a regular basis into repairs. The second owner will get the first repair bill and unload it to the third owner, who will scrap it. It's the circle of life!
Meanwhile the cars that are mostly inherently clean emissions-wise will go on for 200k+ with little more than the occasional spark plugs...
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