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peter scott



Joined: 18 Dec 2007
Posts: 7113
Location: Edinburgh

PostPosted: Fri Nov 04, 2011 10:57 am    Post subject: Reply with quote

Churchill Johnson mentioned fuel saving with speed reduction. Over the last couple of days I was driving from Edinburgh around a number of locations in Lancashire and Yorkshire to visit an engineering shop and a car restorer. Most of the miles were on A class roads and 100 miles of the journey were traveling in convoy with a Riley RMF at 50 mph. My overall average speed was 45 mph according to the computery thing and again use of the brakes was minimal. The overall fuel consumption on this trip of 589 miles was 66.5 mpg, an improvement of 7 mpg over the longer trip that I reported above where my average speed was 49 mph.

Admittedly you aren't going to get fuel economy like this from a Riley RMF with twin SUs. You definitely need the metering control of an electronic fuel injection system. I think the Volvo 940 of the original poster will have fuel injection although I don't know if it's computer controlled.

The horsepower required to propel a car (and therefor fuel usage) at speed has a square law function. Here are graphs showing power required for three different body styles of Jaguar. The solid line is my old SS Jaguar and you can see that it takes 105 bhp to travel at its maximum speed of 87 mph but only half that power to travel at 65 mph.

Peter


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1939 SS Jaguar 2 1/2 litre saloon
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Jim.Walker



Joined: 27 Dec 2008
Posts: 1229
Location: Chesterfield

PostPosted: Fri Nov 04, 2011 5:07 pm    Post subject: Reply with quote

Add a line of wind resistance against speed and you will get a matching curve on your graph.

There are three resistances to motion on a moving vehicle:- wind resistance: rolling resistance; gradient resistance.
Wind resistance is, as your steeply climbing curve on your graph illustrates, a square law and only varies from vehicle to vehicle in relation to its cross-sectional frontal area and its aerodynamic profile .
Rolling resistance changes almost negligibly with speed.
Gradient resistance at constant speed is twice as much on a 1 in 5 (20%)hill as as 1 in 10 (10%), though the actual power required to climb that hill is the same in ft.lbs. for a given load whatever the speed.

Hence wind resistance is the only one we can control. And then only by speed.

The moral is:- keep your speed down and try to keep the engine revs. at the maximum torque of the engine as shown on a torque graph. And don't throw fuel away by slwing down or chaning speed unnecessarily.
Lecture over!
Jim.
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Quote from my late Dad:- You only need a woman and a car and you have all the problems you
are ever likely to want". Computers had not been invented then!
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