engine performance charts (the return)

[joeblogs]

This is a re-post of engine charts I developed last year, but until recently I had no place to host them.

-blogs

I have assembled data on about 80 engines of at least 800 CU displacement produced by about 35 companies in seven nations (France, Germany, Italy, Japan, UK, USA, and USSR). There are many different versions of certain engines (Cyclones, Merlins, Double Wasps, etc), and I have at least some information on about 400 different models.

The source data is a series of books published by Paul Wilkinson (Aircraft Engines of the World) published throughout the 1940s. This source is a little thin on Japanese and Russian engines, so I am looking to augment this with information from other sources (Help!)

[joeblogs]

This figure compares maximum horsepower and engine displacement. There's a pretty obvious linear relationship between the two. Most engine models with the same displacement generate about the same horsepower.



I've identified most of the important engine families on the figure. A vertical sequence of dots typically represents the development of a particular engine (e.g. Merlin or Double Wasp). Some of these sequences are rather large, indicating how far some good designs could be stretched with better fuels, better supercharging, and internal strengthening.

But there are clearly some outliers. These include Continental's X-1430 experimental engine (based on Wright field's "hyper cylinder"), Rolls Royce's Eagle and a few very late model Griffon and Merlin engines. There's also the Napier Sabre VII. All of those are liquid cooled engines. Among the air cooled engines, the outliers include an E model of the Double Wasp, and Wright's turbo compound Cyclone 18.

Speaking of differences between the performance of air and liquid cooled engines, there appears to some systematic differences that vary over engine size:

For displacements under 2000 cu, it appears that liquid cooled engines enjoy a slight advantage over their air cooled brethren.

From 2000-2500 cu it looks like a toss-up.

From 2500-3500 cu, the air cooled engines have the advantage, but there are only a few liquid cooled engines that large.

Among the monster engines--all post war models-- there are two liquid cooled French designs (Arsenal's 24H and Hispano Suiza's 24Z) and two air cooled designs (Pratt and Whiteny's Wasp Major and Bristol's coupled Centaurus). To my knowledge, the only one of these engines produced in quantity was the Wasp Major.

[joeblogs]

This figure shows the relationship between engine horsepower and weight.



Again there is pretty linear relationship between these two variables for most engines. Liquid cooled engines have slight advantage here, and for a few engines a substantial one. They should, as I am using dry weight here (not including the weight of a radiator, coolant, or oil). Including a radiator and coolant should increase the the gross weight of a liquid cooled power plant by 20-40 percent.

There are 33 liquid cooled engines with an HP/dry weight ratio of 1 or higher. These include late war models of the Merlin, Griffon, Allison V-1710, the DB 601E, and the Jumo 213A. The extreme outliers among the liquid cooled engines include a late model Griffon, Napier's Sabre VII, the post war Hispano 12Z, Continental's hyper engine, and the Russian M107.

The 11 air cooled engines with an HP/dry weight ratio of 1 or higher include late war models of the Double Wasp, Cyclone 9, and the post war models of Bristol's Hercules and Centaurus.

[joeblogs]

This figure shows the distribution of Specific Fuel Consumption (lbs of fuel per horsepower per hour) at cruise settings for about 200 engines.



The figure shows that that SFC seems to improve with the octane/performance rating of the fuel (the bars for engines rated on 87 octane avgas tend to be further to the right than the bars for engines rated on 100 octane, and so on). I'm not yet sure whether it's the gas or the vintage of engine that explains the better performance (older engine models are typically rated on lower octane fuels).

The average SFC is about 0.45. Quite a few have a worse fuel economy. Somewhat surprisingly, these consist of many models of the Rolls Royce Merlin and Griffon engines. I had thought that liquid cooled engines would be somewhat more fuel efficient than air cooled engines, but this is often not the case (I think because the best of these are run at higher RPMs).

A number of engines have a better SFC, around 0.42. These consist almost completely of the later versions of Pratt and Whitney's Twin Wasp (1830 cu) and Double Wasp (2800 cu) engines, Bristol's Hercules (2360 cu) and Centaurus (3270 cu) engines, and a few models of Allison's V-1710 liquid cooled engine.

The most fuel efficient engines, with an SFC under 0.40, are the Turbo Compound version of the Wright 3350 and Pratt and Whitney's Wasp Major (4360 cu). Both of these engines were not produced in quantity until after the war.

[joeblogs]

One reason for the higher output to weight ratios of liquid cooled engines is that they are often run at higher RPM. That means more cycles per second of the air pump, which is what an internal combustion engine really is.



Another reason is that, with better local cooling, liquid cooled engines can tolerate higher average pressures in the cylinders (brake mean effective pressure). That means more gas per cycle in the pump, which means more power.

[joeblogs]

This figure compares the amount of output per lb of dry engine weight (specfic power) as a function of the compression ratio of the engine (area in the cylinder with the piston at the bottom of the stroke divided by area of the cylinder with the piston at the top of the stroke).



Note that the German water cooled engines have reletively high compression ratios.

The real outlier in terms of compression ratio is the DB601-N, which I believe was the racing engine that won the Bf109 all its world records. The real outlier in terms of output per weight is Continental's X-1430 which uses the US Army designed hyper cylinder.

For air cooled engines there is a positive relationship between compression ratios and specific power. Note also there is an evolutionary trend towards higher compression ratios as we move from older to more modern air cooled engines.

That pattern is weaker for the water cooled engines where there seems to be a tendency to fix a compression ratio and increase specific power by other means (presumably supercharging). This is particularly true for engines made by Rolls Royce and Allison.

Note that, controlling for compression ratio, the advantage in terms of specific power of liquid cooled engines over air cooled ones is obvious.

[joeblogs]

It was suggested that engines with higher compression ratios are more fuel efficient. This figure plots specific fuel consumption (lbs of fuel per horsepower per hour) against compression ratios.



The figure suggests that any relationship is weak, particularly for water cooled engines. There is some hint of a positive relationship for air cooled engines.

Note that expcept for a handful of models of the Griffon and the V-1710, the engines with the best fuel economy are clearly air cooled.

But we really should control for the type of fuel used, because higher compression ratios usually imply more heat, which requires higher octane fuels. These relationships are brokenout in the following figure.



Controlling for the type of fuel consumed, there does not seem to be any relationship between SFC and compression ratios. The possible exception may be for those engines rated on 87 octane if we focus on the Siddely-Armstrong Cheetah or 91-92 octane if we focus on the Isotta Frashini Delta RC20. But neither of these represent state of the art engines.