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Re: 300 miles 70 mph 30 mpg :)

Subject: Re: 300 miles 70 mph 30 mpg :)
Date: Fri, 22 Dec 95 09:44:58 +0100

> Every engine has a rpm range where it is most efficient. I don't know 
> what that range is on a B engine, and I don't know the rpm that 45mph 
> would correspond to. Especially if your test track involves some slopes, 
> no OD might be better, because I think I remember reading that higher 
> rpm/lower load on the engine gives better mpg (within reason of course).

When I worked for the POD at their research centre in Solihull (late 70's), 
there was a group working on CVTs (Continuously Variable Transmissions). What 
I understood was that, in order to get maximum efficiency, the gear ratio
should be set to run the engine at the lowest RPM at which the required
amount of power could be obtained -- ie. it is much more efficient to 
run at low revs with the throttle wide open, than high revs and and the 
throttle closed off (remember: power == torque x engine speed -- the
idea was to reduce engine speed and maximise torque). 

The setup on the test cars (Triumph Dolomites with 1850 cc engines) could 
raise the gear ratios up to 50mph per 100 rpm. They achieved better 
accelleration and >50 miles per gallon. Unfortunately, the gearboxes also had 
a habit of switching into reverse at 30 mph.....

The greater accelleration was achieved because the gearbox was able to hold the 
engine speed at the peak power point and continuously change the ratio as the
car accellerated. Obviously, there was an electronic controller that had
to understand what the driver wanted from the engine/gearbox setup
(accelleration, economy, etc) at any given instant. 

Overall, it is a great concept, but has bad associations: Daf built cars 
with CVT transmissions which used rubber bands  (well, perhaps they were
really steel) to transfer the power. 


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