8 replies to this topic

[gruntguru]

Here's a good one for all you rev heads. Another thread (Would a turbo-era F1 outrun a modern F1?) gets heated about the real horsepower produced by BMW turbo F1 engines in qualifying trim. For the claims of 1400 - 1500 BHP at 11,000 RPM to be true it has been suggested that the required BMEP of 75 bar is in the same league as Top Fuel dragsters and therefore B.S. since Top Fuel runs Nitro-Methane (90%) which increases output by a factor of 2 or more. Incidentally Top Fuel BMEP is a little higher at 80 - 100 bar.

My initial thoughts were - sure the Nitro is a big advantage, but the F1 has advantages of - better breathing (4 valves/cyl, tuned intake), intercooling, higher boost (5.6 v's 5.0), lower piston speed, no parasitic blower drive, higher compressor efficiency, etc

Let's start with a "back of the envelope" calculation to see what the outer limits for the BMW motor are.

Mass Airflow = Displacement/2 x RPM/60 x V.E. x Air Density x Boost pressure

- Mass Airflow in kg/s
- Displacement in cu metre = 1.5/1000 (/2 in formula gives disp/rev for 4 stroke cycle)
- RPM = 11,000 (/60 converts to revs per second)
- V.E. = Volumetric efficiency = 1.15 (generous - about as good as it gets)
- Air Density = 1.2 kg/cu metre
- Boost pressure = 5.6 bar abs (air density will increase by this factor if intercooler cools air back to ambient - generous)

So airflow = 1.5/2000 x 11,000/60 x 1.15 x 5.6
= 0.8855 kg/s

The maximum fuel that can be burned in this much air is 0.8855/14.7 = 0.0602 kg/s (The engine would actually ingest a lot more fuel than this but all the surplus will pass through the exhaust unburned)

We can now calculate the power input to the engine (total heat energy) assuming a Heating Value for the fuel of 44 MJ/kg.
Heating Power in = fuel flow x H.V. = 0.06024 x 44 = 2.65 MJ/s (MW) (This is 2,650 kW or 3,500 Hp)
Of course only a fraction of this heat is converted to crankshaft power but in round terms, to achieve an output of 1400 hp would require an efficiency of 1400/3500 x 100 = 40%
I'm not sure, but this seems a bit optimistic to me. Bear in mind that this is Oxygen efficiency not overall thermal efficiency since the air-fuel-ratio at this enormous boost would likely be closer to 10:1 for detonation control and internal cooling (remember the plumes of black smoke during qualifying). Assuming 10:1 AFR gives an overall thermal efficiency of 40% x 10/14.7 = 27.2%

Can anyone comment on likely actual efficiencies, poke any holes in my reasoning, suggest modifications to any of the assumptions, etc?

[TDIMeister]

One thing that struck me in all your calculations is that you assume that you're burning gasoline or some such with a stoichiometric AFR of 14.7:1. All it would take for the numbers to look very different is to consider an exotic fuel, such as would have been used in the days of turboed F1 engines back in the day, with a stoichiometric AFR substantially less than this value. The higher the ratio of fuel LHV to stoic. AFR, then for a given mass airflow, the available energy of combustion is higher. You don't need unrealistically high values of thermal efficiency; with fuel enrichment accounted for, a BTE of around 25-27% for that era of engine technology, a figure still hardly changed in the intervening years, is in the correct ballpark.

I recall a post (maybe here, maybe elsewhere) that toluene was used as an additive in F1, itself with a RON of about 121 and MON of 107.

[mariner]

I am not sure about all the details of the fuel used by BMW etc. but was certainly exotic. As well as tackling the octane issue it was blended to acheive a very high specific gravity in order to maximise the energy per litre. This was a response to the fuel tank limit imposed by the FIA in the turbo era.

I would imagine that it was also as oxygenated as much possible in order to increase the pwer potential so it was , in a sense , a little way towards nitro. For qualifying consumption was not an issue. Somewhere there is a wonderul photo of a Brabham BMW on a qualifying run with massive vortices clearly visible off the rear wing because of the black smke from running the mixture over rich.

I think the fuel was deveoped by the BASF chemical company as opposed to a normal petrol supplier.

At the time, as you probably know, a lot of health and safety concerns were raised about the impact on the mechanics etc. of handling it. Simialr concerns led, I beleive to the only US suppliers of Nitro stopping supply to drag teams and thereby leaving Drag racing dependent on Chinese Nitro supplies which caused a potential NHRA crisis when the Chinese plants were shut down to reduce pre Olympics pollution.

[J. Edlund]

The BMW fuel was supplied by Wintershall, the petrol division of BASF. The fuel was also limited by the regs, the maximum octane number was for instance RON 102. Honda published the values 13.7:1 and 41.1 MJ/kg (RON 101.8 and MON 90.0) for its race fuel. Early turbo engines ran on avgas.

These guesstimated power outputs of the F1 engines of the eighties seem to grow over the years, 1200-1300 hp have now become 1400-1500 hp. The power output of the BMW engine was also based on a flash boost pressure reading at the qualifications for the 1986 Monza race; the engine failed just soon after without finishing the lap.

In top fuel the power output is limited by the fuel injection system and the ignition system (limited by the regs). Nitromethane works as a monofuel, meaning that it burns even without oxygen. Due to that, what limits engine power is how much fuel we can supply the engine with, and if we can ignite it.

[gruntguru]

Originally posted by TDIMeister
One thing that struck me in all your calculations is that you assume that you're burning gasoline or some such with a stoichiometric AFR of 14.7:1. All it would take for the numbers to look very different is to consider an exotic fuel, such as would have been used in the days of turboed F1 engines back in the day, with a stoichiometric AFR substantially less than this value. The higher the ratio of fuel LHV to stoic. AFR, then for a given mass airflow, the available energy of combustion is higher. You don't need unrealistically high values of thermal efficiency; with fuel enrichment accounted for, a BTE of around 25-27% for that era of engine technology, a figure still hardly changed in the intervening years, is in the correct ballpark.

I recall a post (maybe here, maybe elsewhere) that toluene was used as an additive in F1, itself with a RON of about 121 and MON of 107.

There are very few fuels in the category you describe TDIM. ie fuels that have an increased heating value per kg of air. Most of them eg nitromethane and nitrobenzene are banned.

Any idea what Lambda value would have applied for the 25-27% BTE's you mention? Certainly the current cars are probably only running 0.9 or leaner which gives a BTE (oxygen) of 27-30% which is way short of the 40% required to give my theoretical BMW its 1400 hp. (I'm starting to agree with McGuire on this one unless someone can point to some assumptions that can be bumped a bit)

BTW is anyone aware of the concept of oxygen efficiency (BTEO) being used out there? I find it useful for this kind of analysis.

[gruntguru]

Originally posted by J. Edlund
The BMW fuel was supplied by Wintershall, the petrol division of BASF. The fuel was also limited by the regs, the maximum octane number was for instance RON 102. Honda published the values 13.7:1 and 41.1 MJ/kg (RON 101.8 and MON 90.0) for its race fuel. Early turbo engines ran on avgas.

Thanks JE. Those figures give a HV (air) of 41.1/13.7 = 3.0 MJ/kg air. This compares to my assumption of
44/14.7 = 2.993 MJ/kg air so there is no benefit in the HV of the fuel. BTW these values are very close to those for Toluene - almost certainly Toluene/N-Heptane as suggested elsewhere in this forum.

[gruntguru]

Originally posted by J. Edlund
In top fuel the power output is limited by the fuel injection system and the ignition system (limited by the regs). Nitromethane works as a monofuel, meaning that it burns even without oxygen. Due to that, what limits engine power is how much fuel we can supply the engine with, and if we can ignite it.

- and whether the engine is strong enough contain the products of combustion?

[cheapracer]

Originally posted by J. Edlund
. The power output of the BMW engine was also based on a flash boost pressure reading at the qualifications for the 1986 Monza race; the engine failed just soon after without finishing the lap.

.

..and because it actually went onto the track makes it historically the most powerful F1 ever regardless of how far it went.

Gruntguru, you didn't mention cylinder size in your small cube f1 vs mega cube drag equation and turbos are parasitic.

Mariner, theres also a picture around of the NME Nissan Rally team crew refueling a Nissan GTI-R in the middle of a forest wearing full spaceman outfit including twin snorkel respirator - also a deadly blend.

[gruntguru]

Originally posted by cheapracer

Gruntguru, you didn't mention cylinder size in your small cube f1 vs mega cube drag equation and turbos are parasitic.

Agree - small cylinder size is a big advantage. (must have come under "etc" in my original post)

Parasitic MEP for a well designed turbo system is a small fraction of that of a Roots blower at the same boost. Even a crankshaft driven centrifugal is far less efficient than the exhaust driven equivalent.