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yahoo2

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Joined: 05/04/2011
Location: Australia
Posts: 1166

Posted: 05:57am 24 Oct 2011

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I watched Jeremy Clarkson test drive the 1000 H.P. Bugatti Veron in Europe on the TOP GEAR show, while driving at about 150Kmh He looks at the dash and comments
, " At the moment we are using...... 50 Horsepower......so that's......nine hundred and fifty to go then! So he promptly floors the throttle.

It got me thinking , what does it take to cruise at the speed limit?

I have always been a bit nerdy about fuel economy and my current car is not fantastic in this department, it feels heavy. I got a 7.3 L/100Km on the last tank but it was hard work due to the fly by wire throttle. My best in my old ford Ute at the speed limit is 6.1 L/100Km for a 1600Km round trip.

Anyway, I had a look on the internet and found a formula and instructions for calculating cruising kilowatts, so Ive had the GPS and stopwatch out every time that I found a empty straight road for the last month.

the result 17154 watts.........23 HORSEPOWER!!

that seems like a lot to me, it doesn't include the engine systems and torque converter. There is probably another 7 H.P. hiding around the motor.

I don't get it! Where does it all go?

I tried the same test with an old jap diesel Ute at 110 Kmh and I get 9 HORSEPOWER !!

yahoo

I'm confused, no wait... maybe I'm not...

VK4AYQ
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Joined: 02/12/2009
Location: Australia
Posts: 2539

Posted: 10:16am 24 Oct 2011

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Hi Yahoo

It is a very difficult thing to work out as there a so many variables involved, over the years I have done a lot of checking to work out a rule of thumb figure and come up with 4 HP per liter for an efficient naturally aspirated engine and 4.5 hp per liter for a turbocharged engine under the same test conditions.

A diesel engine will do 5 HP per liter and for a turbo inter-cooled engine up to 6.5 HP liter.

However a lot depends on the cam design and running the engine at the peak efficiency band on the cam and also the size of the engine as a smaller engine generally runs at a wider throttle setting to produce the same power as a larger one, this results in a higher effective compression ratio allowing the fuel to burn more efficiently.

In a diesel engine the engine runs at maximum compression ratio at all times allowing for a better fuel burn, and when turbo boosted the MEP is much higher again so more efficient.

My car a V6 commodore has a long term average fuel usage at 100 KLM of 8 ltr per 100 K which means a fuel burn HP ratio of 32 horsepower on the average. If I could cruise at 115 KLM hr it drops to 6.5 Ltr per 100 klm, this is because it is running in a more efficient rev range on the cam. Lots of variables.

The roll resistance of the car comes into play as well as there is a certain effort required to move a car on flat ground, this is related to Tire type and Tire pressure and as speed increases wind drag becomes a significant power drag, more variables.

All the best

Bob

Foolin Around

yahoo2

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Joined: 05/04/2011
Location: Australia
Posts: 1166

Posted: 04:05am 25 Oct 2011

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I basically got the car 10k over my target speed then kicked it into neutral and timed it to 10k under and used the deceleration to work out the energy that I am losing.

So basically I am measuring most of the resistance and friction involved.

Its just messes with my head that something that looks so slippery and sleek is so hard to push. Yet my old banger that looks like a besser block glides along like an ice skater.

I know the large radiator and the underside of the car catches a lot of wind and these sticky grippy low profile tyres don't help, as well as the air that tears off of the sloping rear window and the turbulence from the non functional spoiler.

yahoo

I'm confused, no wait... maybe I'm not...

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