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[geoffr70]

Hello, as above really.

Don't really know why, I've never had a diesel, didn't know they had vac pumps til I replaced the leaking gasket on my dads transit.

Is it just because they have less airflow or something so don't create as much vac? Just guessing!

Also, just to confirm, I would've thought the vacuum would be at its highest under high load, and lowest at idle, but someone told me the opposite, which is it?

Thanks

[Kevin Wood]

Petrols are vacuum pumps, so an external one is not needed. Vacuum is present in the intake manifold except when engine load is high.

Diseasels don't have throttles, so the intake system doesn't generate significant vacuum. In order to use the same ancillaries (brake servo, heater controls, etc.) most manufacturers drive a small vacuum pump from the engine.

Kevin

[geoffr70]

thanks

[Debs.]

That`s also why a diesel`s 'engine-braking' is far less than with a petrol engine; which acts like an air compressor on the over-run.......but in a diesel, the over-run compression acts like a pneumatic spring, with the cylinder`s trapped-air being first compressed and then re-expanding within the cylinder after each cycle, thus the diesel engine`s weak-retardation is mostly via internal thermo-frictional losses.

The 'Jacobs brake' (aka: Jake-Brake) was developed to make more effective use of diesel engine braking, by making changes to the engine`s valve operation/timing.....and what a noise they make when operating!

http://www.jakebrake.com/products/how-the-jake-brake-works.php

[TheBoy]

Lack of engine braking most my excuse for eating pads every 7k on a crappy diesel escort that work gave me (they didn't realise they one lasted 3.5k, as I changed every other set).

The mechanics disagreed, and tried telling me a diesel had MORE engine braking. Retards.

[tidla]

diesels do have more engine braking than petrol due to the higher compression ratio.

[Andy B]

diesels do have more engine braking than petrol due to the higher compression ratio.

that was my impression when driving my TD Astra, but I can only compare to an auto Omega V6

[Debs.]

diesels do have more engine braking than petrol due to the higher compression ratio.

Erm, no they don`t......although it`s a common misconception!

Superior compression ratio isn`t the factor in over-run braking; moreover it`s related to the diesel`s lack of 'intake throttling' and valve timing/action which differs from a petrol engine.

That`s why the diesel 'exhaust-brake' and 'Jacobs Brake' were invented!.....valves are additionally re-opened to expell compressed cylinder-air on the over-run cycle, rather than that energy being returned to the piston.....such 'Jake Brakes' are extremely effective retarders, but only in that way can the 22.1 compression ratio be exploited effectively as an engine brake.

[aaronjb]

diesels do have more engine braking than petrol due to the higher compression ratio.

that was my impression when driving my TD Astra, but I can only compare to an auto Omega V6

Automatics have basically zero engine braking though, because the drive train is (essentially) decoupled from the engine - although the Omega occasionally locks up the torque convertor on long periods of overrun (e.g. downhill) and then you get a feel for something more akin to the real engine braking of the engine (though it still feels awfully soft, I presume an amount of it is still absorbed in the transmission itself)

[Omegatoy]

diesels do have more engine braking than petrol due to the higher compression ratio.

Erm, no they don`t......although it`s a common misconception!

Superior compression ratio isn`t the factor in over-run braking; moreover it`s related to the diesel`s lack of 'intake throttling' and valve timing/action which differs from a petrol engine.

That`s why the diesel 'exhaust-brake' and 'Jacobs Brake' were invented!.....valves are additionally re-opened to expell compressed cylinder-air on the over-run cycle, rather than that energy being returned to the piston.....such 'Jake Brakes' are extremely effective retarders, but only in that way can the 22.1 compression ratio be exploited effectively as an engine brake.

Debs, you never cease to amaze me!!! been a class 1 hgv driver for 30 odd years, jake brakes and exhaust brakes are second nature to me, and i couldnt have explained that any better

[tidla]

diesels do have more engine braking than petrol due to the higher compression ratio.

Erm, no they don`t......although it`s a common misconception!

Superior compression ratio isn`t the factor in over-run braking; moreover it`s related to the diesel`s lack of 'intake throttling' and valve timing/action which differs from a petrol engine.

That`s why the diesel 'exhaust-brake' and 'Jacobs Brake' were invented!.....valves are additionally re-opened to expell compressed cylinder-air on the over-run cycle, rather than that energy being returned to the piston.....such 'Jake Brakes' are extremely effective retarders, but only in that way can the 22.1 compression ratio be exploited effectively as an engine brake.

yes youre right.
any advantage before tdc is lost after tdc