Your thoughts please

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Ok wammers, I don't think your the type to admit when your wrong. A BSc (hons) & 20 years design engineering may not meet with your status but I know enough of the basics.:p:
 
martynv8 said:
Ok wammers, I don't think your the type to admit when your wrong. A BSc (hons) & 20 years design engineering may not meet with your status but I know enough of the basics.:p:
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Not enough i am afraid. Obviously you never had anything to do with diesel engines or engines of any description. Take the EDC engine out of a P38 diesel and put in a 4.4 V8 using the same transmission. Even with it's common rail system which improves efficiency considerably i am willing to bet it uses more fuel than the 2.5 did in the same car. A 4.4 litre diesel engine running at the same RPM as a 2.5 litre engine WILL use more fuel. More markedly a 4.4 litre petrol engine running at the same RPM as a 2.5 litre petrol engine WILL use more fuel. There is a fuel air mixture ratio to be maintained, larger cylinder capacity, more air, more fuel simple as that. When you design your perpetual motion machine let me know i would be interested to see it.
 
wammers said:
Not enough i am afraid. Obviously you never had anything to do with diesel engines or engines of any description. Take the EDC engine out of a P38 diesel and put in a 4.4 V8 using the same transmission. Even with it's common rail system which improves efficiency considerably i am willing to bet it uses more fuel than the 2.5 did in the same car. A 4.4 litre diesel engine running at the same RPM as a 2.5 litre engine WILL use more fuel. More markedly a 4.4 litre petrol engine running at the same RPM as a 2.5 litre petrol engine WILL use more fuel. There is a fuel air mixture ratio to be maintained, larger cylinder capacity, more air, more fuel simple as that. When you design your perpetual motion machine let me know i would be interested to see it.
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So, if you are saying RPM is the governing factor, how is it that any engine will use more fuel going up hill than it does on the flat at the same RPM?
 
wammers said:
Not enough i am afraid. Obviously you never had anything to do with diesel engines or engines of any description. Take the EDC engine out of a P38 diesel and put in a 4.4 V8 using the same transmission. Even with it's common rail system which improves efficiency considerably i am willing to bet it uses more fuel than the 2.5 did in the same car. A 4.4 litre diesel engine running at the same RPM as a 2.5 litre engine WILL use more fuel. More markedly a 4.4 litre petrol engine running at the same RPM as a 2.5 litre petrol engine WILL use more fuel. There is a fuel air mixture ratio to be maintained, larger cylinder capacity, more air, more fuel simple as that. When you design your perpetual motion machine let me know i would be interested to see it.
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:your_wrong:
 
Datatek said:
So, if you are saying RPM is the governing factor, how is it that any engine will use more fuel going up hill than it does on the flat at the same RPM?
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That is about load Keith, throttle has to be increased to maintain the same RPM under load as opposed to running light, trying to keep things simple. As Ant said the mathematics needed to allow for all conditions are horrendous. If you had two similar design motors, one 500 cc running at 2500 RPM and next to it a 1000 cc engine running at 2500 RPM. Each connected to it's own one gallon fuel tank. The 1000 cc engine would run out of fuel first every time. If the 500 cc engine was running at 5000 RPM they would run out at about the same time.
 
martynv8 said:
:your_wrong:
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Why would i do that when i am not? The vehicles in question is a P38 Range rovers, to avoid complications all are on a flat road not going up nor down hill. All P38 Range rovers have the same gearbox and final drive ratios. Are you following up to now? So at any given speed above torque lock all the engines, 2.5 diesel, 4.0 and 4.6 litre petrols are doing the same RPM. Got that? Forget the diesel we all know that will be using less fuel than the other two. Agreed? Now then we get to the petrol engines. We have a 4.0 and a 4.6 litre petrol engine turning for the purpose of this illustration at 2500 RPM. Both are inducting air and being fueled to maintain 2500 RPM. Because each 4.6 litre cylinder is marginally larger than the 4.0 litre cylinder it has more air in it, therefore it needs more fuel to maintain the mixture ratio of around 14 to 1 than the 4.0 litre unit does. So it uses more bloody fuel. Did you follow that.
 
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wammers said:
That is about load Keith, throttle has to be increased to maintain the same RPM under load as opposed to running light, trying to keep things simple. As Ant said the mathematics needed to allow for all conditions are horrendous. If you had two similar design motors, one 500 cc running at 2500 RPM and next to it a 1000 cc engine running at 2500 RPM. Each connected to it's own one gallon fuel tank. The 1000 cc engine would run out of fuel first every time. If the 500 cc engine was running at 5000 RPM they would run out at about the same time.
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I think you're oversimplifying things a bit there for the real world as an engine's power curve usually isn't linear (eg with boats (can you tell I love boats :p ) the fuel burn per hour - which I think is a better measure than mpg as it's independent of hills and mass etc - is usually more than double at full throttle (6250rpm on mine) than 3000ish rpm.)

Surely this doesn't need to be this difficult though - it's about energy; there's a finite amount of energy in 1 gallon of fuel. Variations in mpg or gph between engines depend on how efficiently they extract that energy into something useful (ie not just noise and heat like the Rover V8 does :p ). It therefore seemingly makes sense that since the 4.6 is really just a bigger version on the 4.0, it would use the same amount of fuel at the same road speed because the amount of energy being used to move the car is in theory the same, and the figures basically reflect this. However given that there is more stuff moving around in the bigger engine it must use slightly more energy.
This works with considering rpm difference as at higher rpm there is more useless work being done actually in the engine moving things around faster, even if there's theoretically the same load on the engine (same road speed etc) as at lower rpms - ie why higher gears are more economical.
Comparing it to a TDV8 isn't the same as it's capable of extracting more useful energy from the fuel based on design, but I do not see how that can be the case with the 2 Rover V8s given that 1 was a re-drilled version of the other. I suppose there could be an argument that if more power is delivered at lower rpms there is less useless work going on.
 
TCubed said:
I think you're oversimplifying things a bit there for the real world as an engine's power curve usually isn't linear (eg with boats (can you tell I love boats :p ) the fuel burn per hour - which I think is a better measure than mpg as it's independent of hills and mass etc - is usually more than double at full throttle (6250rpm on mine) than 3000ish rpm.)

Surely this doesn't need to be this difficult though - it's about energy; there's a finite amount of energy in 1 gallon of fuel. Variations in mpg or gph between engines depend on how efficiently they extract that energy into something useful (ie not just noise and heat like the Rover V8 does :p ). It therefore seemingly makes sense that since the 4.6 is really just a bigger version on the 4.0, it would use the same amount of fuel at the same road speed because the amount of energy being used to move the car is in theory the same, and the figures basically reflect this. However given that there is more stuff moving around in the bigger engine it must use slightly more energy.
This works with considering rpm difference as at higher rpm there is more useless work being done actually in the engine moving things around faster, even if there's theoretically the same load on the engine (same road speed etc) as at lower rpms - ie why higher gears are more economical.
Comparing it to a TDV8 isn't the same as it's capable of extracting more useful energy from the fuel based on design, but I do not see how that can be the case with the 2 Rover V8s given that 1 was a re-drilled version of the other. I suppose there could be an argument that if more power is delivered at lower rpms there is less useless work going on.
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A larger cylinder needs more air to fill it the more air you have the more fuel you need to maintain the mixture ratio that applies throughout the rev range. It's as simple as that. Unless you get into variable cam timing and **** like that every engine has a sweet range it likes to play at. 2T outboards drink fuel for fun flat out. That is because they have twice as many power strokes as a four stroke does. Of course the more load you put on the motor the more the throttle has to be opened to maintain RPM but that is to complicated to work out it's a bloody nightmare scenario thinking about it. But the 4.6 has to use marginally more fuel than the 4.0 at steady speeds it can't do a lot else. If the ratios were different and the 4.6 was doing 2000 RPM at 60 MPH and the 4.0 litre doing 2500 RPM at 60 MPH of course it may be slightly better. But that is not the case.
 
wammers said:
A larger cylinder needs more air to fill it the more air you have the more fuel you need to maintain the mixture ratio that applies throughout the rev range. It's as simple as that. Unless you get into variable cam timing and s*** like that every engine has a sweet range it likes to play at. 2T outboards drink fuel for fun flat out. That is because they have twice as many power strokes as a four stroke does. Of course the more load you put on the motor the more the throttle has to be opened to maintain RPM but that is to complicated to work out it's a bloody nightmare scenario thinking about it. But the 4.6 has to use marginally more fuel than the 4.0 at steady speeds it can't do a lot else. If the ratios were different and the 4.6 was doing 2000 RPM at 60 MPH and the 4.0 litre doing 2500 RPM at 60 MPH of course it may be slightly better. But that is not the case.
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I'm always talking about 4 stroke outboards as that's what I have, I'd never go back to 2-strokes after the simplicity of my Tohatsu :)

I agree with you completely; I was going off Autotrader's figures for the 4.0 vs 4.6 which do indicate a slight improvement on the 4.6 and which I assuuuuumed are literature values.. But digging around they seem to have made them up as other equally official looking sources suggest the inverse! Should've trusted my instincts it seems haha

Irrespective of everything though people, get the diesel! :p
 
TCubed said:
I'm always talking about 4 stroke outboards as that's what I have, I'd never go back to 2-strokes after the simplicity of my Tohatsu :)

I agree with you completely; I was going off Autotrader's figures for the 4.0 vs 4.6 which do indicate a slight improvement on the 4.6 and which I assuuuuumed are literature values.. But digging around they seem to have made them up as other equally official looking sources suggest the inverse! Should've trusted my instincts it seems haha

Irrespective of everything though people, get the diesel! :p
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It's hard to believe you think a four stroke outboard is a simple engine. Nothing like as simple as a 2T. But the four stroke will be a lot less thirsty. I had a three cylinder Suzuki 85 on my boat, it drank juice for fun if you opened it up. I thought of getting the new 115 four stroke but it was not over here then. Looked a US sites and it was $7,999.00 about £4500.00 over there at that time. When it became available here it was £7,999.00 so somebody is making a lot of money somewhere.
 
wammers said:
It's hard to believe you think a four stroke outboard is a simple engine. Nothing like as simple as a 2T. But the four stroke will be a lot less thirsty. I had a three cylinder Suzuki 85 on my boat, it drank juice for fun if you opened it up. I thought of getting the new 115 four stroke but it was not over here then. Looked a US sites and it was $7,999.00 about £4500.00 over there at that time. When it became available here it was £7,999.00 so somebody is making a lot of money somewhere.
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Haha well certainly not simple mechanically but, it ALWAYS works!
My Johnson 150 was mechanically trivial but reliability was a tad iffy and it kept requiring bits of work. Not to mention the 0.5 mpg equivalent on the boat it was on :p
The little Tohatsu 30 is a beautiful piece, never let me down :)
 
TCubed said:
Haha well certainly not simple mechanically but, it ALWAYS works!
My Johnson 150 was mechanically trivial but reliability was a tad iffy and it kept requiring bits of work. Not to mention the 0.5 mpg equivalent on the boat it was on :p
The little Tohatsu 30 is a beautiful piece, never let me down :)
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Sounds like a Thai bride, you ought to get one Wammers, to go with the Swedish nurse.:D:D Mind you 30 is a bit old:)
 
Wammers in my mind is quite right...in lab conditions with no external loads or anything both engines running at the same RPM the larger capacity engine has to use more fuel to keep the Stoich mixture correct....(this is where it tries to maintain a 15:1 Air/Fuel mixture) i.e. 15x more air than fuel by mass, this creates the optimum burn ratio to fully combust the fuel to extract the most amount of energy per unit volume.

The 4.6 litre engine has a capacity of .6 unit volume more than the 4.0 so if we are to do some simple maths (cos we don't do enough maths if you ask me) to maintain a 15:1 stoich mixture....:

4.0 litres of air needs 0.267 litres of fuel to burn efficiently
4.6 litres of air needs 0.307 litres of fuel to burn efficiently..

the difference in % terms is about 13%

Now this is were the fun comes in....

The % difference in real world terms between the two varies from 3-5% based on the LR supplied data for Urban, Extra Urban and Combined figures....the reason for this is the fact the 4.6 can extract more torque per cylinder explosion and whilst most bang on about how much BHP their car produces BHP is feck all compared with torque...torque is a measure of wrok efficiency...how well the engine can generate power at a given speed and fuel/air mixture....the fact the 4.6 generates torque more efficiently than the 4.0 is what closes the real world fuel consumption gap from 13% to 2-5%....for each unit volume of fuel burnt the 4.6 can more efficiently use that energy created from burning the fuel to provide torque....

BUT in our Lab conditions above...where we aren't using the torque generated, as the engines have no load on them so therefore torque is effectively zero....two engines running at the same speed with differing capacities, the larger capacity will consume more fuel...
 
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