tegwin
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| posted on 4/12/07 at 10:19 PM |
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An unanswerable question on efficiency..
I had a thought today....(not often it happens!)...
If I had a 6 Litre V8 engine producing...lets say 400Hp....
And a 1.8 litre 4 pot with a supercharger and clever injection producing the same 400Hp
Which engine would consume more fuel to produce peak Hp...
My somewhat nieve view is that, to produce 1 hp you need to burn x amount of fuel...
So 400hp requires X amount of fuel regardless of how big your engine..
Any clever bods care to comment?
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JoelP
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| posted on 4/12/07 at 10:24 PM |
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depends which engine is more efficient! the v8 has more friction etc, more coolant to move round, more oil to move round, more weight to spin round
etc. Its off to a bad start!
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twybrow
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| posted on 4/12/07 at 10:24 PM |
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I would say the supercharged 4 pot would use less, as in theory it is more efficient?! But that would apply more to a turbo... So ummmm, not really
sure.
There are lots of factors affecting fuel economy: air/fuel ratio, carbs/efi, running temperature, friction and a load more that I have no idea of!
If you do indeed have both engines, stick the supercharged lump into your car (I'm sure you could get a good price for the V8 on ebay) and give
me first ride please!
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oadamo
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| posted on 4/12/07 at 10:27 PM |
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i had a cossie that put out 300bhp and i could get my mpg down to 20- 24 miles to a tenner. so either one you will be skint lol.
adam
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JoelP
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| posted on 4/12/07 at 10:37 PM |
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i once got 90 miles out of £40 with the 300zx, that made me feel ill!
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graememk
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| posted on 4/12/07 at 10:39 PM |
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i've just done 540 miles on a tank full
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smart51
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| posted on 4/12/07 at 10:42 PM |
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The amount of power that gets to the flywheel doesn't just depend on the amount of fuel you burn.
The higher the compression ratio, the more energy from the fuel gets pushed into the piston. Those clever bods from Yamaha can run their engines at
12:1 and more and yet still run on 91 octane fuel.
Swirl and combustion chamber geometry affect how complete the combustion is. If you only half burn your fuel, you're not getting all the energy
out of it.
Spark timing sets when the hot gasses start to push on the piston. Too soon and you slow the piston down on the compression stroke. Too late and
your power goes straight out of the exhaust.
Then there's all the power the engine wastes sucking in air and blowing out exhaust, plus friction and ancillaries.
In general though, turbos are more efficient for the same power than NA engines. Look at the 2.0 KV6 vs the 1.8T in the rover 75. That said, you
could turbo an old dog and it still not be as good as a modern efficient block.
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onzarob
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| posted on 4/12/07 at 10:42 PM |
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quote:
Originally posted by tegwin
I had a thought today....(not often it happens!)...
If I had a 6 Litre V8 engine producing...lets say 400Hp....
And a 1.8 litre 4 pot with a supercharger and clever injection producing the same 400Hp
Which engine would consume more fuel to produce peak Hp...
My somewhat nieve view is that, to produce 1 hp you need to burn x amount of fuel...
So 400hp requires X amount of fuel regardless of how big your engine..
Any clever bods care to comment?
Its the same fuel and Air which causes the same bang!!!
It generally considered to match a blown engine with a naturally aspirated engine youe need twice the capacity 
1.8 T x 2 = 3.6L
V8 6L x 1 = 6L
so not an easy one, i would say the 6l would drink more, due to greater friction and losses and size.
so if it was a3.6l v8 would it drink more.....I would say yes still...same reasons
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mistergrumpy
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| posted on 4/12/07 at 10:44 PM |
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Ah but how big is your tank - not to get too personal
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martyn_16v
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| posted on 4/12/07 at 11:29 PM |
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If you can find the data, what you're looking for is the brake specific fuel consumption (BSFC), in simple terms it's the amount of fuel
an engine uses to produce a horsepower. It's not a fixed figure for an engine, the BSFC will vary across the engines operating range. As has
already been mentioned, there are a great many variables which affect how efficient an engine is.
Speaking of which, has anyone seen the new HCCI
engine Merc are working on? If that takes off it could be the death of diesel engines in cars
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tegwin
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| posted on 4/12/07 at 11:54 PM |
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Hmmm thats interesting...
That reminds me....Clarksons comment on TG about that Hydrogeon car producing nothing but water...making it a 0 emmisions car...
Where does he think the energy comes from to produce the hydrogeon in the first place...
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Chippy
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| posted on 5/12/07 at 12:12 AM |
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Don't know about car engines, but marine engines are reconned to consume 5 gals per 100 HP, at full chat, and that is regardless of capacity or
being turbo'd. (It does not apply to two stroke motors, as these are very ineficiant, consumption wise) Boat fitted with 2 X 1000 HP Mans uses
100 gallons per hour, so about £500 per hour running flat out, (NICE). Cheers Ray
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RazMan
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| posted on 5/12/07 at 08:36 AM |
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quote:
Originally posted by tegwin
If I had a 6 Litre V8 engine producing...lets say 400Hp....
And a 1.8 litre 4 pot with a supercharger and clever injection producing the same 400Hp
I'm not sure about the fuel efficiency, but I know which one is going in my next project
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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MikeRJ
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| posted on 5/12/07 at 08:48 AM |
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quote:
Originally posted by smart51
In general though, turbos are more efficient for the same power than NA engines. Look at the 2.0 KV6 vs the 1.8T in the rover 75. That said, you
could turbo an old dog and it still not be as good as a modern efficient block.
In general a turbo charged petrol engine is LESS efficient.
Off boost they are disadvantaged by low compression ratio, on boost they suffer from increased pumping losses compared to a N/A engine.
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smart51
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| posted on 5/12/07 at 10:17 AM |
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quote:
Originally posted by MikeRJ
quote:
Originally posted by smart51
In general though, turbos are more efficient for the same power than NA engines. Look at the 2.0 KV6 vs the 1.8T in the rover 75. That said, you
could turbo an old dog and it still not be as good as a modern efficient block.
In general a turbo charged petrol engine is LESS efficient.
Off boost they are disadvantaged by low compression ratio, on boost they suffer from increased pumping losses compared to a N/A engine.
Conventional wisdom disagrees with you.
Smart don't have a small turbo engine just to put the cost up. Audi's TT 1.8T is more efficient than the V6, as is the Rover 75. Small
turbo diesel engines do better than larger non turboed diesels, for the same power.
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Ivan
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| posted on 5/12/07 at 02:55 PM |
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^^^ I agree with smart51.
The smaller Turbo motor will generally be more economical on the road in normal light throttle driving due to much lower friction and inertia, pumping
and other losses than the larger na motor that is required to produce the same power.
At light throttle the restriction of the turbo is effectively zero as all the engine sees is the total restriction of the throttle plate and turbo
which means you need less throttle restriction for the same vacuum. Its impact on pumping losses is thus minimal.
[Edited on 5/12/07 by Ivan]
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cossey
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| posted on 5/12/07 at 07:03 PM |
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in general large engine are more efficient in that they use less fuel per hp hence why the most efficient piston engines are house sized diesels
turning at 100rpm.
i would expect that if you put both engines on a dyno and ran them constantly at 400bhp output the v8 would use quite alot less fuel but the overall
fuel consumption of both when placed in identical cars could be quite similar as the v8 will suffer high pumping losses at lower outputs and the 4 pot
would be less efficient at higher loads.
if you look at the bsfc figures for many tuned v8s youll will be surprised that in many cases they are extremely efficient and can rival even modern
diesels.
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MikeRJ
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| posted on 7/12/07 at 01:15 PM |
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quote:
Originally posted by Ivan
^^^ I agree with smart51.
The smaller Turbo motor will generally be more economical on the road in normal light throttle driving due to much lower friction and inertia, pumping
and other losses than the larger na motor that is required to produce the same power.
At light throttle the restriction of the turbo is effectively zero as all the engine sees is the total restriction of the throttle plate and turbo
which means you need less throttle restriction for the same vacuum. Its impact on pumping losses is thus minimal.
BUT at light throttle the low CR hurts thermal efficiency. Given two otherwise identical engines, one with a low CR and one with a high CR, the high
CR engine will always be more efficient. A Turbo charged engine only has a high CR when under boost, and this is when pumping losses start
hurting.
A double wammy is that turbocharged engine normally have to run rich when under boost in order to cool pistons and avoid detonation.
[Edited on 7/12/07 by MikeRJ]
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Liam
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| posted on 7/12/07 at 01:44 PM |
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OP says supercharged - not turbocharged, and supercharged is way less efficient than turbocharged being driven off the crank. I'd go for the V8
being more efficient at 400bhp than the supercharged 1.4, especially if it's allowed to be something like a nice modern LS2 V8 (bang on 6 litres
and 400bhp incidentally - anyone got a BSFC figure?).
liam
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02GF74
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| posted on 7/12/07 at 04:07 PM |
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quote:
Originally posted by smart51
The higher the compression ratio, the more energy from the fuel gets pushed into the piston. Those clever bods from Yamaha can run their engines at
12:1 and more and yet still run on 91 octane fuel.
I agree with smart here.
To keep it simple, let's assume the engine cylinder plus components are the same on both engines.
I've read the Vizard article and efficiency has a lot to do with about CR which translates how much pressure there is pushing the piston as it
moves down the bore.
So I would tend to favour the s/c engine.
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cossey
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| posted on 8/12/07 at 10:54 AM |
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cr does effect efficiency but it is by no means the only factor. a 1.8l engine to produce 400bhp is going to require alot of boost and even with a
very efficient screw supercharger the blower will need 75bhp to run it so the engine in reality has to produce 475bhp at the crank.
the dynamic compression ratio of a tuned n/a engine and a f/i one are very similar.
i did a quick simulation of both engines using engine analyzer and the s/c 1.8 came out at having a bsfc of around 0.48lb/bhp/hr where as the v8 came
out at 0.45.
the 4 pot was pratically a race engine before the supercharger went on though so with like for like treatment the v8 would be nearer 0.4 all be it
producing far more than 400bhp. the ultra tune race v8s are getting down to nearly 0.35 which highlights the point that race engines tend to be far
more efficient than road engines.
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