I tend to only "engine brake" in a low gear when going down hills and trying to hold the car back without needing to brake. I try to anticipate speeds and minimise braking. Braking is a waste of fuel, but sometimes it can't be helped 
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driving and the use of brakes
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Posted 6 months ago #
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A STWer yesterday...
www.youtube.com/watch?v=b7ll_s3ka88Wheres the flower?!
Posted 6 months ago # -
I'm surprised that gearboxes can handle the stresses of harsh acceleration with aplomb, but not the stresses of deceleration. Unless you're dumping the clutch at redline revs, I'm struggling to see how engine breaking can be any worse than vigorous driving.
Posted 6 months ago # -
Ah yes... the air charge drawn into a petrol engine under almost closed throttle will be very low pressure, so the piston will be sucked up instead of working against atmospheric pressure. What's the actual inlet manifold pressure differential?
On the subject of compressors, it has occurred to me that a diesel compressing the same amount of air all the time is very wasteful considering it only uses a little bit of that air. So it would be muc more efficient to have a small V8 and under normal driving leave one side of it unused and leave the valves open, so you're not compressing any air. Large American petrol trucks do this so the tech exists.
Posted 6 months ago # -
Never seen the words V8 and efficient used in the same sentence before, unless 'not' is in there too... Lol! I'm pretty sure some big eighties American saloon cars did the deactivating of cylinders thing, (not one bank, that would be horrible, the front four I think) but I don't think it caught on. Good idea in principle but complicated in execution, you need separate camshafts for the valves of the cylinders that will be deactivated and the engine needs to be configured in such a way as when running on less cylinders it isn't horribly out of balance.
Posted 6 months ago # -
It has caught on in the trucks. And the reason for de-activating one bank is that it's far easier to do - just have a clutch on one of the cam pulleys I suppose.
Anyway I am talking about a 2.0l V8, say, which would give normal performance and good economy.
Posted 6 months ago # -
These were banned as they sound like a machine gun.
close but not quite. the machine gun noise has to be silenced, but lorries still have Jake brakes.Of course diesels have engine braking. anyone who lets off the accelerator in any diesel vehicle knows that. but its not the same as a petrol, where the braking is naturally through the throttle. it is "artificially induced" through either a throttle on the exhaust side of things (increasing pressure on the upstroke), or on turboed vehicles, closing the turbo off to restrict the flow of air into the cylinder in the first place (creating a vacuum on the intake stroke)
pretty sure you can hear the rattle of pressure release valves whenever I let the foot off the accelerator in my HDi, though cant be certain. i might look into it...
Posted 6 months ago # -
Anyway I am talking about a 2.0l V8, say, which would give normal performance and good economy.
Yeah I reckon that would be feasible, but would be a very expensive engine to produce. Two cylinder heads, clever gubbins and lots to go wrong, not to mention very small components with high stress loads. Easier to develop the inline 4. Which is what has happened, largely.
"artificially induced" through either a throttle on the exhaust side of things (increasing pressure on the upstroke), or on turboed vehicles, closing the turbo off to restrict the flow of air into the cylinder in the first place (creating a vacuum on the intake stroke)
Give over! What would be the possible incentive to engineer a complex system of shutting off turbos or extra valves just to provide a little diesel engine with engine braking, which IT ALREADY PRODUCES IN SPADES because it is a diesel and has a compression ratio of 20:1!!! A jake brake is only neaded when holding back 40odd tonnes down extended grades and you don't want to overheat the road brakes. Your HDi Does. Not. Have. an additional engine braking system, although of you can hear rattling on the overrun I would agree that you should look into it, or chop it in quick...Are you trolling me??? Lol
Posted 6 months ago # -
You really don't get how a compressor works do you? have you ever seen one when it is off load? they turn really easily. An engine when open to the atmosphire and driven from the crank shaft squezzes the air which is then free to return to the volume it was before before its pussed out into the atmosphere. On a car you have friction and an amount of restriction from an exhust but this is the same regradless of fuel source. A compressor sucks air and the compresses this into either a system or reciver just like pumping air into a tyre. the air has no where to go and this makes it hard the higher the pressure.
my 205 gti wouldn't have had any engine braking due to my foot being on the clutch liek the teach in the driver test.
Also how come its fine for you to compare landys with different egines but not for any one else?Posted 6 months ago # -
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.
taken from herePosted 6 months ago # -
my 205 gti wouldn't have had any engine braking due to my foot being on the clutch liek the teach in the driver test.
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Posted 6 months ago # -
IanMunro - Member
my 205 gti wouldn't have had any engine braking due to my foot being on the clutch liek the teach in the driver test.
!?
As part of the emergency stop you have to press the clutch in and not stall the car by just jumping on the brakes as it shows a lack of control or sum such thing and doing this in the 205 which had a tick over of 850 rpm and the fuel would only cut in at 1000rpm it had the habit of stalling its self.Posted 6 months ago # -
On the subject of compressors, it has occurred to me that a diesel compressing the same amount of air all the time is very wasteful considering it only uses a little bit of that air.
You need all that air to compress to create heat to combust the fuel, as I said before, the energy is recouped on the combustion stroke, probably to a greater degree than in a petrolas well as you're warming up the air which expands against the piston, the aste heat in a petrol goes into the cyliner walls (hence why petrols warm up quickly).
The V9-V4 thing gets done quite alot, didn't cadilac have a concept for a 16cyl engine where the cylinders came on sequentialy? The ballance isnt a problem as you just make a V4, ballance it, then replicate it up the engine (same way as porche eventualy ballanced their 8cyl race engines in the 902*)
*google says I'm wrong, but it was one of their first successful endurance race cars, the one with the stingray mouth.
v8ninety - Member
If diesel is so good why is your username V8ninety, not 2.5-4cl-na-diesel-ninety?
Posted 6 months ago # -
Also how come its fine for you to compare landys with different egines but not for any one else?
Compare what you want, so long as its comparable. Landys are what I know, and have experience of. In this situation they are ideal because the 2.5 diesel and petrol engines are based on the same engine block, same bore and stroke, with different heads and pistons to alter the compression ratio. The gearboxes, transferboxes and differentials are identical ratios in both petrol and diesel models of a certain era, which make them ideal 'control' vehicles in this instance.
If diesel is so good why is your username V8ninety, not 2.5-4cl-na-diesel-ninety?
Never said diesel was better, love me V8! But its cheaper, and the engines have more rotational resistance, leading to more engine braking. It's probably something to do with why they produce more torque at comparable revs, too...Posted 6 months ago # -
Maybe a straight 6 with a clutch to make it two sets of three?
Posted 6 months ago #
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