Lack of Engine Braking

tedted

think of the exhaust brake on a diesel not used much now but was very common a ew years ago before brakes improved aa lot

PasturesNew

It's probably just because of where you're used to driving... as the only hill in the whole county is the 400 yards descent from the Shire Hill down to the traffic lights, real hills will have made your brain think "Ooh 'eck" (yes, your brain changes accents when you drive) .... ".... this is scarey/different...."

It's just typical flat-lander thinking when presented with an actual hill.

Joe_Horner

Compression actually has very little effect on engine braking, which is why diesels (with huge compression ratios) need exhaust brakes or compression-release ("Jake") brakes.

In the absence of a throttle, the cylinder takes a more-or-less full charge of air, which is compressed like a spring as the piston rises. But, as the piston passes TDC and enters what would be the power stroke, most of the energy used in compressing the air is given back to the piston by the air expanding.

Imagine a bike pump with your finger over the outlet. Pressing the handle in requires effort but,if you then let go of the handle, it springs back out sharpish thanks to the potential energy in the compressed air.

There's a small amount lost to heating and friction effects but even most of the heat is recovered as the expanding air cools.

System

Joe_Horner wrote: »

Compression actually has very little effect on engine braking, which is why diesels (with huge compression ratios) need exhaust brakes or compression-release ("Jake") brakes.

Nope. They need them because the vehicles weigh 44 tonnes so every bit of braking helps.

Exhaust brakes work better the higher the engine revs. With most trucks now being automatics, if you descend a hill with the exhaust brake on it'll change down gears so the engine is running just below the redline. Quite effective too, the Euro 6 DAF CF I used to drive could hold its speed at 40 tonnes descending from Stainmore on the A66 on just the exhaust brake alone but it was revving its nuts off 100RPM below the redline.

DoaM

BeenThroughItAll wrote: »

No, the information comes from 30 years of understanding how an engine works, I'm afraid, but Wikipedia should reflect the same information.

ETA: Unless I don't understand and I'm wrong as I've said below.

I was indicating that Strider's info may have come from there. Apologies for any confusion.

BeenThroughItAll

DoaM wrote: »

I was indicating that Strider's info may have come from there. Apologies for any confusion.

No worries. That's clear when I read it again. I think I'm probably wrong anyway!

DoaM

EssexExile wrote: »

Drive a traditional automatic the you get used to little or no engine breaking.

I didn't realise that engines in automatic vehicles were near-enough bullet-proof!


Oh, you meant braking.

nickcc

tedted wrote: »

think of the exhaust brake on a diesel not used much now but was very common a ew years ago before brakes improved aa lot

Exhaust brake on a diesel just blocked the exhaust down pipe, Cummins Jake brake turned the engine in to a compressor by changinging the exhaust valve opening times.

Joe_Horner

Tarambor wrote: »

Nope. They need them because the vehicles weigh 44 tonnes so every bit of braking helps.

That's a factor with lorries, but not with all the diesel cars (especially non-turbo) that have used them over the years.

Muscle750

My Audi A6 3.0 TDI auto i just drop the cogs on the padals on the steering wheel going down steep hills and it will hold its speed no probs.