109 e3b against spitfire II

[Kurfürst]

Originally Posted by Viper2000

Because the supercharger work is W*Cp*deltaT, you will find that injecting fuel into the eye of the supercharger results in a considerable reduction in the supercharger power required for any given boost pressure, which naturally improves brake power and brake SFC.

Pretty redundant if you ask me, if you are already using a variable speed supercharger adjusted already for supercharging needs, as DB did with its barometrically controlled hydraulic clutch.. the supercharger wasn't making less waste, it made almost no waste at all.

Correct me if I am wrong, but injecting fuel into the supercharger eye reduces charge temperature, delaying detonation point. Direct fuel injection does the same (in the combustion chamber), but later, and with better fuel effiency, no risk of backfires, and no negative G problems. High octane fuel is a pretty expensive agent for charge cooling..

[Viper2000]

You're confusing throttling losses, which are avoided by varying supercharger speed, with the aerodynamic losses associated with the supercharger's design, which are not.

The aerodynamic losses increase the temperature rise associated with a given pressure rise, which increases the work required.

Because compression through a given pressure ratio tends to produce a given temperature ratio, you reduce the absolute deltaT by reducing the initial temperature, all other factors remaining constant. This reduces compressor work, which is equivalent to an increase in compressor efficiency.

You obviously get a greater benefit from cooling the working fluid at the start of the compression process.

Eg

Start at 288 K. Cool by 25 K. T = 263 K. Compress through a temperature ratio of 1.5, and then further through a temperature ratio of 2. Temperature = 263*1.5*2 = 789 K. Delta T from start = 501 K.

Compare with:

Start at 288 K. Compress through temperature ratio 1.5, cool by 25 K and then compress through temperature ratio of 2. Final temperature is then 814 K, so delta T is 526 K.

Thus, compressor work differs by 5% or so in this example.

This is the reason for the reduction in the isentropic efficiency associated with a given polytropic efficiency as compressor pressure ratio increases.

[Crumpp]

Direct fuel injection does the same

Injecting into the eye of the supercharger is no more efficient than a carburetor at fuel metering.

Eg

Start at 288 K. Cool by 25 K. T = 263 K. Compress through a temperature ratio of 1.5, and then further through a temperature ratio of 2. Temperature = 263*1.5*2 = 789 K. Delta T from start = 501 K.

Compare with:

Start at 288 K. Compress through temperature ratio 1.5, cool by 25 K and then compress through temperature ratio of 2. Final temperature is then 814 K, so delta T is 526 K.

Thus, compressor work differs by 5% or so in this example.

You are still getting the compression from a supercharger and directly cooling the combustion chamber with direct fuel injection.

I am sorry Viper but it does not change the fact you cannot precisely meter the fuel thru an intake, either.

Modern engine designs are now starting to employ direct-injection.
Here fuel is introduced straight into the cylinder rather than
being premixed outside. This technique has major advantages
as it allows more accurate control of the quantity of fuel used and
the injection timing, leading to an increase in the vehicle’s
horsepower combined with a lower fuel intake.

Direct-injection is far more efficient than traditional
injection techniques, raising performance levels while requiring less fuel.

http://www.onsemi.com/site/pdf/PSDE_0411.pdf

[Viper2000]

Originally Posted by Crumpp

Injecting into the eye of the supercharger is no more efficient than a carburetor at fuel metering.

Of course it's better than a carburettor; you're not reliant upon venturi suction to get the fuel into the airflow; you're positively chucking it in with a pump.This will provide considerably better matching across the flow range.

Because you're injecting fuel under pressure, you can positively atomise it, achieving a considerably lower Sauter mean diameter of fuel droplets than is possible with a simple carburettor, which means that it will evaporate much faster. This means that more of the temperature drop happens earlier in the compression process, which increases the overall efficiency bonus.

Once the fuel has evaporated, the mixture distribution problem goes away.

Originally Posted by Crumpp

You are still getting the compression from a supercharger and directly cooling the combustion chamber with direct fuel injection.

This is inherently less efficient. It's relatively simple thermodynamics.

Originally Posted by Crumpp

I am sorry Viper but it does not change the fact you cannot precisely meter the fuel thru an intake, either.

Fuel metering is not a particular problem. You measure the flow through the intake, and inject fuel in proportion thereto. It's much easier to get this measurement & injection process right at a single point than it is to get it right at 12 points.

Fuel distribution may be a problem at low manifold temperatures where the fuel fails to evaporate fully, but that is a separate problem.

You seem to be mostly hung up on fuel metering issues, which certainly exist for naturally aspirated engines with carburettors, especially away from their design point.

However, fuel metering is not especially important at high power if you don't care about emissions. You run rich of stoichiometric, and fuel flows say +/- 5% won't make a great deal of difference to power output. Obviously the SFC is pretty bad at that point; you can clearly see this on the SFC curve in Lovesey's paper.

Mixture distribution is not a problem at high induction manifold temperature.

The reductions in supercharger work and intercooler size are far more important than the slight increase in fuel mass fraction which you might suffer from the need to keep the leanest cylinder sufficiently rich to avoid detonation. The cost of a single point system is far lower than a multi-point system, and the fuel pressure required is lower than for true direct injection. (Port injection is a pretty horrid compromise which only makes sense if the alternative is a carburettor which would produce bad mixture distribution.)

For a supercharged spark-ignition aero-engine, operating at a fixed non-dimensional power setting, provided that you've got enough induction manifold temperature to avoid condensation, the mixture distribution will be good and the single point system wins.

Multi-point FI is an expensive solution to mixture distribution problems. It is great for naturally aspirated engines, and probably pragmatic for turbo-normalised engines, especially if the engine manufacturer isn't responsible for the turbocharger.

But if you're using a mechanical supercharger and will mostly operate the with reasonably high induction manifold temperatures, then there's no great mixture distribution problem unless your induction manifold is horrible, so multi-point injection offers limited benefit, whilst single point injection into the eye of the supercharger reduces supercharger drive power requirements. So single point injection is a pretty obvious choice.

Now, if you're designing a sports car engine, you might supercharge it to get high power, but most of the time it would operate at very low non dimensional power settings, so mixture distribution would be a major problem with single point injection, and therefore you'd probably go for multi-point FI.

But that's because the sports car engine isn't really designed for high performance. It's designed to make an expensive noise and very occasionally provide bursts of acceleration to impress the girl in the passenger seat. Most of the time it's practically ticking over, and so you're much more bothered about part-load characteristics than would be the case for an aero-engine. You're also trying to meet modern emissions regulations, which means that you're paranoid about stoichiometry so that you don't poison your catalyst. It's a totally different world, with different trades and different drivers.

[TomcatViP]

Originally Posted by Viper2000

But that's because the sports car engine isn't really designed for high performance. It's designed to make an expensive noise and very occasionally provide bursts of acceleration to impress the girl in the passenger seat.

I hve to disagree loudly with the above comment. It's the worst thing to do Having a fat diesel with a body kit is what impress the girl nowadays

[Crumpp]

http://www.slideshare.net/rjperforma...rj-performance

It works in any engine to increase power and performance over any other fuel metering system no matter if the intake is supercharged or not.

http://www.steerbythrottle.com/hccyong/files/DFI.pdf

Advantages of direct fuel injection

Combined with ultra-precise computer management, direct injection allows more accurate control over fuel metering (the amount of fuel injected) and injection timing (exactly when the fuel is introduced into the cylinder). The location of the injector also allows for a more optimal spray pattern that breaks the gasoline up into smaller droplets. The result is more complete combustion -- in other words, more of the gasoline is burned, which translates to more power and less pollution from each drop of gasoline.
Disadvantages of direct fuel injection

The primary disadvantages of direct injection engines are complexity and cost. Direct injection systems are more expensive to build because their components must be more rugged -- they handle fuel at significantly higher pressures than indirect injection systems and the injectors themselves must be able to withstand the heat and pressure of combustion inside the cylinder.
How much more powerful and efficient is direct injection?

Cadillac sells the CTS with both indirect and direct injection versions of its 3.6 liter V6 engine. The indirect engine produces 263 horsepower and 253 lb-ft of torque, while the direct version develops 304 hp and 274 lb-ft. Despite the additional power, EPA fuel economy estimates for the direct injection engine are 1 MPG higher in the city (18 MPG vs 17 MPG) and equal on the highway. Another advantage: Cadillac's direct injection engine runs on regular (87 octane) gasoline. Competing cars from Infiniti and Lexus, which use 300 hp V6 engines with indirect injection, require premium fuel.

http://cars.about.com/od/thingsyoune...tinjection.htm

A good primer article on Direct Injection. It explains very well the difference between the various types of fuel injection and why Direct Injection is the ultimate fuel metering system for power and performance.

http://www.driverside.com/auto-libra..._injection-350

Here is a good article that explains the German Direct injection systems in easily understandable terms:

http://www.motorcycleproject.com/mot...xt/inject.html

The German systems were far from perfect but they certainly did their job and allowed them to level the playing field in terms of aircraft engine performance.

IIRC, the British and United States did a combined effort to develop a Direct Injection engine that was used in a tank at the end of the war.

[Crumpp]

You seem to be mostly hung up on fuel metering issues

Viper, direct injection is a fuel metering system. Supercharging is an intake system and does not have a thing to do with fuel metering.

It is two completely separate things you want to combine.

You point to the thermal benefits of introducing the fuel ahead of the supercharger instead of downstream of it. I agree it is there when compared to introducing fuel downstream of the supercharger in your INTAKE SYSTEM.

Got it but that does not make it more thermally more efficient that directly injecting the fuel in the combustion chamber. You are not looking at the heat capacity but are stuck on upstream vs downstream fuel introduction for your intake.

As Rolls Royce points out, direct injection is much more efficient than metering your fuel downstream or anywhere in the intake system. Just because that small part of the intake system becomes more efficient vs introducing fuel downstream does not make the whole system more efficient.

A simple illustration of that basic principle.

1006 J/kgC – Specific Heat Capacity of Normal Air

460 J/kgC – Specific Heat Capacity of Steel

2100 J/kgC – Specific Heat Capacity of Gasoline

To change the temperature of a mass of 1 Kg of each by 2 degrees….

Air = 1006 J/kgC * 1kg* 2 C = 2012J
Fuel = 2100J/kgC*1kg*2 C = 4200J
Steel = 460J/kgC * 1kg * 2 C = 920J

Our 4200J of fuel energy goes to cool the 15C air…

4200J * 1kg /1006J/kgC = Change in T = 4.17 C

15C - 4.17C = 10.83C

Why do you think direct injection is the ultimate fuel metering technology and so desirable to have in an engine? If was not for the complexity and expense, all engines would be direct injected because it is the most efficient system mankind knows of at the present for metering fuel.

Rolls Royce also shows in the article you posted their system is not as efficient as directly injecting fuel into the combustion chamber combined with your supercharged intake.


Why? YOU STILL MUST HAVE A FUEL METERING SYSTEM ON YOUR ENGINE. Introducing the fuel ahead of the supercharger does not eliminate the basic problem of NON DIRECT INJECTED FUEL METERING SYSTEM, uneven fuel mixtures found by introducing fuel ANYWHERE in the intake system.



The only way to eliminate that is too directly inject fuel into the combustion chamber.

Direct injection engines with a supercharger STILL benefit from that supercharger intake system.

[Crumpp]

Doesn't a supercharger (or turbocharger for that matter) run as a single intake, and then splits a tube towards each cylinder?

Yes and you are correct in that metering fuel anywhere in the intake system is not going to give you the power gains compared to injecting precisely metered fuel directly into the combustion chamber.

The drawbacks to direct injection are the expense and complexity. The BMW801 series used 14 high pressure fuel pumps and consisted of more parts than the entire rest of the engine.

The supercharger technology of the allies combined with better fuels restored the balance.

You cannot point to Direct Injection technology and say it was decisive and gave the German engines better performance over the allied ones anymore than you can point to better fuel quality and supercharger technology of the allies as being better than the German engines.

In the air war over Europe, all sides developed their engines and fielded 2000 hp plus designs by the wars end. The implications made a few folks that fuel technology was decisive are not correct when one takes in the whole picture. Fuel quality and supercharger technology simply maintained the balance with fuel metering technology as well as superior chemical engineering.

That any of these engines were routinely operated outside of their published guidelines is another ludicrous idea hatched in the gaming world but that is another subject.

[Viper2000]

Originally Posted by cheesehawk

Doesn't a supercharger (or turbocharger for that matter) run as a single intake, and then splits a tube towards each cylinder?

If that's the case, isn't fuel/air being routed towards each cylinder even while that cylinder isn't in its intake cycle? How could adding the fuel pre-supercharger give as precise a charge to each cylinder, when its being routed towards many cylinders that aren't prepared to accept the charge, as a direct injection which pumps it directly into the cylinder head only when the cylinder is needing it?

Air is a mixture of gasses which have different molecular weights. But you wouldn't expect that the issues you allude to above would modify the composition of air going into the cylinders.

If the fuel has evaporated, and is in a gaseous phase, which is perfectly reasonable if the induction manifold temperature is high, the exact same argument applies; inertial separation of species within the gaseous mixture is unlikely because the forces are insufficient to overcome the diffusive tendency of the gas.

Originally Posted by cheesehawk

All the math and numbers aside, yes, your air/fuel mix via a supercharger/turbocharger, is mixed well, but you're still backing it up against a closed valve for 3/4 of the cycle, then pushing this in when the valve opens. Also, backpressures and eddies in the airflow probably effect the droplets of fuel, allowing them to consolidate (albeit very little) before the valve opens. This might add to the compression, which might help power output, but surely can't be great for precise fuel metering.

If the fuel is still liquid in the intake manifold then mixture distribution will be bad. This is often the case with naturally aspirated engines because the induction manifold temperature is essentially ambient.

The benefit of single point injection into the eye of the supercharger is that it improves the effective isentropic efficiency of the supercharger, and thus reduces supercharger work, increasing the brake horsepower output of the engine when compared with multi-point injection into the cylinders or ports.

I'm getting pretty tired of trying to explain this relatively simple concept.

Here's a nice big paper. It's got a lot of interesting stuff in it, but the bit that is germane to this discussion is the analysis of the effects of water injection upon compressor performance. It doesn't matter whether you're injecting fuel or water, the latent heat reduces the temperature rise through the compressor, which is analogous to an increase in compressor efficiency.

Happy reading.