----- Original Message -----
From: <wwd@netheaven.com>
To: "Military Vehicles Mailing List" <mil-veh@mil-veh.org>
Sent: Friday, October 05, 2001 8:19 AM
Subject: Re: [MV] "Jake Brakes" Possible on an M35?
> In <list-1322082@skylee.com>, on 10/05/01
> at 03:19 AM, "Richard Notton" <Richard@fv623.demon.co.uk> said:
>
> >Its simpler than you think, just a butterfly valve in the exhaust
> >manifold usually operated by an air cylinder from the vehicle brake
> >supply.
>
> But that is not going to actually release the cylinder pressure to the
> atmosphere. Thus the following downstroke will simply rebound the energy
> stored up during compression and you gain zero 'engine braking', save for
> friction losses.
>
Errr, no. This is only true when the pressure in the exhaust system is
sufficient to overcome the force of the exhaust valve spring, in practice
the butterfly valve is designed to be imperfect.
> Now, if you added a throttle/choke plate to the intake, as in a gasoline
> engine, you can very simply derive the same measure of 'engine braking'
> that a gas engine offers. Which is not inconsiderable. Plus it would serve
> as an emergency anti-runaway device should the injection pump ever fail.
> Nice to have if you go down steep mountain roads.
>
> The Jake brake system must, somehow, kick open the exhaust valve BTDC to
> release the compression and then immediately close it, to create suction
> on the 'power' stroke. Alternatively, a poppet valve could be added to the
> combustion chamber and, timed accordingly, offer the same effect.
>
You need to think "suck, squeeze, bang, blow", the exhaust valve opens
around BDC or just before and pushes a lump of air into the exhaust system
which is trapped by the butterfly valve, at TDC or just after the exhaust
closes, the induction stroke takes on another cylinder full of air,
compresses and releases this, then also pushes this off down the exhaust
pipe, we now have two strokes of air per cylinder compressed in the exhaust
system.
Without firing any charge, the four stroke engine is a compressor but with a
wasted compress and release stroke.
Richard
Southampton - England
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