109 e3b against spitfire II

[TomcatViP]

Originally Posted by Viper2000

OTOH, because the supercharged engine is smaller, it has less non-cooling drag, and so you'd expect to cruise faster, which helps to make the radiator smaller.

Funny that Merlin Vs DB tell the opposite.

Seems you forgot one parameter : rpm and charging raise the strain and the temp with negative consequence on efficiency: try to win the 24h Le Mans race with a 2L engine and then jump in 7.0L 'vette

To put it in perspective : there was no successful post war Merlin engined airliner. But lot of with P&W primitives big radials

[Viper2000]

Originally Posted by TomcatViP

Funny that Merlin Vs DB tell the opposite.

Seems you forgot one parameter : rpm and charging raise the strain and the temp with negative consequence on efficiency: try to win the 24h Le Mans race with a 2L engine and then jump in 7.0L 'vette

To put it in perspective : there was no successful post war Merlin engined airliner. But lot of with P&W primitives radials

rpm isn't especially interesting. Piston speed is generally a better metric.

Liquid cooled engines run colder than air cooled engines, and actually one of the main problems for the Merlin was over-cooling of the charge during cruising flight, which necessitated modification of the aftercooler to act as a heater to prevent the charge temperature falling below 40ºC.

The Merlin powered version of the DC-4, the Canadair Northstar was considerably faster than its radial engined equivalent. Noise was a problem initially due to the stub exhausts; the big radials tended to have collector rings; a crossover exhaust for the Merlin mitigated this to some extent. It wasn't an unsuccessful machine, but it wasn't ever going to capture the US market because it wasn't American.

As for perspective, how many DB powered airliners were there post WWII?

The Merlin wasn't successful as an airliner engine for many reasons - it hadn't be designed for that sort of duty for a start. It did rather better than the V-1710 though.

But perhaps the main reason for its "failure" as an airliner was that there just weren't suitable British airliners to bolt it onto. Lancastrian, York & Tudor could hardly compete with contemporary products from Lockheed & Douglas, because Britain had basically stopped airliner development in 1939 whilst the Americans had continued throughout the War (because they needed long-range transports anyway). They weren't about to put British engines onto their aeroplanes if they could possibly help it, so the considerable technical lead of the American airframers translated directly into market share for their engine manufacturers.

It's probably better to compare the Merlin's civil record with that of Hercules & Centaurus, which faced a similar airframe problem (though of course at this time Bristol had an aeroplane division as well, which provided them with a captive market for their engines).

In this context, the Merlin doesn't look so bad.

[CaptainDoggles]

Originally Posted by Viper2000

As for perspective, how many DB powered airliners were there post WWII?

I suspect there is politics involved with that.

[Viper2000]

Quite.

To a lesser degree the same argument applies to British engines, given that the most successful airliner airframes were American in 1945. Therefore comparison between the Merlin and the R-2800, R-3350 or R-4360 in the civil market isn't really fair; it makes more sense to compare it with the Bristol Hercules or Centaurus, and if you perform that comparison then the Merlin doesn't look quite so much of a "failure" in the civil market anymore...

[Crumpp]

I suspect there is politics involved with that.

It was....

Both Daimler-Benz and BMW were forbidden from even being in the aviation market.

Post-war, both companies withdrew from anything to do with aviation and produced automobile engines instead. Both are industry leaders from the moment they entered the market and that leadership continues today.

They produced some of the best engines in the world.

Injecting fuel upstream of the supercharger reduces the temperature by about 25 K due to the latent heat of evaporation of the fuel.

And injecting fuel directly into the combustion chamber is even better, Viper. How hard is that to understand?

If you reduce the power consumed by the supercharger then you increase the brake horsepower and reduce the SFC.

And that it is much more efficient to realize the power gains by directly injecting fuel into the combustion chamber than it is by dumping it into an intake manifold......

It's less attractive for a big aero-engine because if you're operating at fixed power with a reasonable amount of supercharge you should be able to attain excellent mixture distribution, and so the pragmatic solution is to have single point injection into the eye of the supercharger - which is basically what everybody ended up doing.

No, it is attractive and if we had the technology to do it on a cost effective basis, we would have done it. It is the ultimate fuel metering method for a piston engine in terms of power and efficiency. A single point injection simply cannot maintain a stoichiometric mixture in all the cylinders. That is why the EGT and CHT will always be different in each cylinder unless you have direct fuel injection.

[Crumpp]

In fact in the 1950's, we started doing it.....

In the R-4360C Wasp Major power-plant with CH 9 turbo-blower.....

The most important differences between the BH 4 installation (Fig. 3)
and the new power-plant (Fig. 4) are the elimination of the
intergral mechanically-driven supercharger and the carburettor
in the former in favour of the direct fuel injection and
no secondary (internal) supercharger in the latter.

http://www.flightglobal.com/pdfarchi...0-%200248.html

[Viper2000]

Originally Posted by Crumpp

In fact in the 1950's, we started doing it.....

In the R-4360C Wasp Major power-plant with CH 9 turbo-blower.....



http://www.flightglobal.com/pdfarchi...0-%200248.html

The benefit comes from getting more work out of the turbocharger; if you cruise higher then the turbocharger has a bigger expansion ratio to work with and can therefore get more work out of the exhaust.

Charge temperature is limiting, and you'd obviously rather get your supercharger work from the turbine than the crankshaft. So you throw away the supercharger, but that means you need to either go to DI or else inject into the eye of the turbosupercharger impeller to homogenise the mixture.

The turbocharger came from GE, whilst the piston engine came from P&W.

Fuel injection into the turbocharger wasn't viable because of the fire risk, both in case of leaks between the hot and cold sides of the turbocharger, and because of the relatively long ducting from turbocharger to piston engine, which would otherwise have been full of stoichiometric mixture. But most importantly, it wasn't viable because it would have been almost impossible to start the engine unless the turbocharger was clutched to the crankshaft for that purpose, which in turn wasn't possible due to the physical separation between turbocharger and piston engine which was itself a consequence of the historical decision that GE would make turbochargers in isolation from the piston engine manufacturers.

The thermodynamic benefit comes from utilisation of exhaust enthalpy which would otherwise have gone to waste. However, there is an enthalpy loss equal to the sum of enthalpy drop across the aftercooler, and the cooling drag on the cold side thereof; if fuel had been injected upstream of the turbosupercharger, the compression process would have had a higher apparent isentropic efficiency, and the aftercooler would have had less work to do because the compressor delivery temperature would have been lower.

In essence, the benefit comes from improved matching/work balance rather than from going to DI itself. In other words, they wanted to throw away the supercharger to get more of their compressor work from the turbocharger, and this drove them to DI because they then didn't have a method to homogenise the mixture. So DI is a consequence rather than a cause.

[Viper2000]

Originally Posted by Crumpp

It was....

Both Daimler-Benz and BMW were forbidden from even being in the aviation market.

AFAIK they had to quit for 3 years. Thereafter they didn't get back into aerospace because they didn't have a market rather than anything else.

However, Daimler has quite a big stake in EADS, whilst BMW started a joint venture with RR to make turbofans in Germany from 1990, though now this is 100% owned by RR.

Originally Posted by Crumpp

And injecting fuel directly into the combustion chamber is even better, Viper. How hard is that to understand?

Why do you think it's better?

A supercharger is a pretty effective way to homogenise a mixture. The intake manifold is going to end up at roughly charge temperature, which for a Merlin at high power is going to be about 90ºC. You are very unlikely to see condensation of the fuel onto the manifold at that temperature. FAR will therefore be pretty constant from one end of the manifold to the other.

Charge distribution may well vary, which would modify CHT somewhat, but the same argument applies to air distribution.

FAR will become variable when supercharger delivery temperature is low, and this will affect acceleration behaviour, especially from low boost & revs. But aero-engines spend most of their time at fixed, relatively high, power settings, and so this sort of transient behaviour is far less of a problem for an aero-engine than for a car engine.

Originally Posted by Crumpp

And that it is much more efficient to realize the power gains by directly injecting fuel into the combustion chamber than it is by dumping it into an intake manifold......

If you are supercharging then you'll win by injecting into the supercharger and thereby reducing supercharger work.

The supercharger is basically adiabatic if you're not injecting fuel or water into it. However, isentropic efficiency of superchargers tends to be much lower than the isentropic efficiency of the compression stroke of a piston engine.

In any case, you're always going to gain more by reducing temperature as early in the compression process as possible, because compressors (whether steady-flow or non-flow) produce temperature ratios in exchange for pressure ratios, whilst the absolute work required for the compression process is proportional to deltaH, i.e. Cp*deltaT.

If you reduce the starting temperature then you reduce the deltaT all the way down the chain, and the benefit multiplies. Therefore, if your fuel is liquid, you really want to inject it at or before the start of the compression process in order to maximise the thermodynamic benefit associated with its latent heat of evaporation.

Clearly for a naturally aspirated engine you might as well go for direct injection, especially if the number of cylinders is small.

The cylinders & pistons are very far from being adiabatic, but are very efficient at performing compression work. The limiting factor is the rate at which they can pass non-dimensional flow through their intake & exhaust valves at any given rpm. Hence supercharging; pre-compressing the air allows you to get more absolute mass flow rate into the fixed non-dimensional mass flow capacity of the piston engine. That's the objective of the exercise.

You use a steady flow machine upstream of the unsteady flow machine because unsteady flow machines are inherently bigger than steady flow machines, and therefore you can shrink the physical size of the engine in relation to its effective flow capacity.

Originally Posted by Crumpp

No, it is attractive and if we had the technology to do it on a cost effective basis, we would have done it. It is the ultimate fuel metering method for a piston engine in terms of power and efficiency. A single point injection simply cannot maintain a stoichiometric mixture in all the cylinders. That is why the EGT and CHT will always be different in each cylinder unless you have direct fuel injection.

That's only true for naturally aspirated engines.

EGT and CHT will be different anyway because that's life; holding FAR constant is great but it's not magic; airflow into the cylinder depends upon induction manifold design and engine speed. Induction manifold design is quite a complex business, and compromises are inevitable.

DI is very useful if you want to vary non-dimensional power setting over a wide range, but this isn't so important for an aero-engine, and so the higher design-point efficiency offered by injecting into the eye of the supercharger is a pretty compelling argument, before you even consider the cost, mass and complexity advantages.

Modern GA engines are going DI because they're going CI (in order to burn Jet-A and save money), and also because they don't have a lot of cylinders, which means that the cost of injectors is inherently less important.