langwadt wrote:321apex wrote:langwadt wrote:
yes WRC runs a 34mm air restrictor on the inlet, limiting the power to 300 something or there about
F1 runs a 100kg/h fuel restriction limiting the power to 500-600 depending on how they can use it
in both cases it has the effect of cutting the top of the normally peaky power curve of a highly tuned engine
making it mostly flat in the range where an unrestricted engine could have made more power but is limited
by the fuel or air limit
Forgive me for saying it explicitly, but it is rather misguided to compare air limited WRC engine to fuel limited F1 engine. Those two methods of power limitation are quite different and carry with them different operational and performance implications. At this point, since we clarified this, I would agree with the statements you made if we are speaking about an orifice restricted engine - with boost you can move pwr/tq characteristics.
My remarks however about relatively "immovable" nature of TQ and PWR peaks along RPM axis concerned the type of engine that is not air restricted and as such it responds to operational factors differently. F1 engines ARE NOT restricted by an orifice and I maintain what I said prior in reference to that.
If necessary I'd be glad to explain why.
yes please explain
- WRC rally formula is based upon restricting the air flow RATE into the engine.
- F1 limits the fuel flow RATE as well as the QUANTITY consumed.
These are fundamental differences, which set the optimization logic and consequently control methods apart.
- WRC car does not carry the air on board and since it is not limited by the amount of fuel it burns, it can be wasteful with the fuel. Typical WRC rally car with 34mm restricting orifice produces theoretically 300HP at it's peak RPM, but it can make 400HP or more at lower engine speed. Comparing both instances, approximately the same amount of air and fuel is used with 25% less power output. When you look at BSFC when 300HP are produced vs 400HP you notice how great is the efficiency difference all due to pumping losses.
- F1 car has to carry the whole fuel amount on board. If the engine is especially efficient the competitor may gain a weight benefit of carrying less of it for the race distance. How much is 10 kg worth a lap ? 0.2-0.4s on average I think. Monza could be challenging to even finish the distance with allotted fuel.
- WRC drives on public roads, carries little speed around corners, has 2 less gears and must have a much wider operating rev range 2000-7000 RPM, which is about 71% of total 0-7000.
- F1 car will use no more than 3000 RPM operational window and if we assume 12500 RPM as a practical operational limit, then it will operate within 9500-12500RPM widow, which is 24% of of total operating range.
- WRC engineering job is to play out all tricks out of the toolbox (including wasting fuel), to achieve the sort of boost characteristics using, spark retardation, etc. to always have the traction/driveability demanded amount of boost on tap in this 2000-7000 RPM range
- F1 engine must always operate at it's most efficient possible BSFC
This is why, fundamentally the rev range in F1 will not be arbitrary and they will either make the engine run all day at 12.5k or 15k RPM - but not both. The only way to achieve optimum, average BSFC for the operating rev range is to FIX this range and subordinate all other aspects of the engine to it.
In closing, I would like to draw your attention to some fundamentals which guide the preformance limits of any engine that combusts oxygen from the atmosphere.
As you know engines are air limited pumps. "Choked flow" sets the limit to ingest air.
http://en.wikipedia.org/wiki/Choked_flow
Specifically in Otto cycle engines, there is a key relationship between the flow of air thru the curtain area between valve and seat & piston speed on it's way down the intake stroke. This piston speed is proportional to the engine speed. You can read more about it in the following link on pages 252-254:
http://books.google.pl/books?id=UtxI5gX ... ne&f=false
In cylinder head design, when the sizing selection takes place, certain parameters must be established in hardware, which will later support the sort of performance we are seeking. Specifically, when a duct of an inlet/exhaust port is being defined, its flow diameter is established as a consequence of target flow velocities as well as mass flow rate. Too big or too small of a port and you loose. Consequently, the operating engine speed is a fundamental parameter around which the valve diameter and shapes, cam timing and lift are decided upon.
So this is why, IMHO
the engine makers will pick a specific operating rev range of their engines and optimize the BSFC within that range to maximize fuel efficiency.
