War Boost: An Introductory Lesson In WWII Aircraft Engine Supercharging

In so many ways as we have discussed on this channel before, World War II was a war of horsepower.

It was a war of octane, a war of boost, a war of durability, and a war of innovation.

Among the most fascinating parts of that war to examine are the elements surrounding piston

engines, especially the liquid cooled ones used in military aircraft. So many of the bedrock

principles of high performance engines that we know today were explored to incredible scientific

depth while trying to create airplanes to fly higher, faster, and more efficiently than those of

the enemy. While the massive air-cooled radial engines are in so many ways the most brutally

powerful units bolted into planes during the conflict, they don't quite spin my crank in ways

that the liquid cooled engines do. A few videos back when we went deep into the early history of

centrifugal superchargers in drag racing, I mentioned that I was going to be exploring

superchargers over the course of a few different pieces. Now consider this the second one of those.

Because this is such a big topic and because there's so much to cover, I'm going to use this video

as an introductory level look into the various different methods of supercharging used on the

most popular liquid cooled airplane engines of World War II. We're talking about the American

Allison V1710, the German DB600 series of engines, and of course the British designed and engineered

Rolls-Royce Merlin. In following videos down the road, I'm going to go deeper into the engines,

the people, and the performance that they yielded, but this is interesting nonetheless to examine.

Many of you are likely gearheads with the love of these historic machines and this may be stuff

you already know, but as I have learned, the audience that watches this stuff is often made of

younger enthusiasts who are learning the principles of engines and the history that surrounds the

horsepower we can make today. If you know all this stuff already, awesome. If you don't,

I hope you dig it. More depth will come in time as we dive into various different aspects of the

stuff we cover here. I used resources like A History of Aircraft Piston Engines by Herschel Smith,

Allison Power of Excellence, and Airplane Power from 1943, published by General Motors in the

making of this video. Now when it comes to supercharging, each country had its own idea of

the best method. They all have their pluses and minuses, and as we'll see, one country out of the

three chose a path that ended up being a pretty significant mistake. For starters, we should

probably examine why a piston aircraft engine needs a supercharger to be effective at high altitudes.

As we all know, engines need oxygen to properly combust fuel, and the higher we get above the

level of the sea, the less of it there is in the atmosphere. By using a supercharger to ram

massive quantities of air into the cylinders of an engine, as opposed to allowing it to get there

by the engine inhaling it naturally, the same engine that would be choking and failing at altitude

can now produce horsepower almost equal to what it would make on the ground.

If that seems like a weird principle, think of it this way. As the supercharger is cramming

all that additional volume of air into the cylinders, it allows the same amount of fuel

to be burned, or close to it, that would be burned at sea level, effectively tricking the

engine into thinking that it's down in better air. Of course, it's easy to understand the

basics here, but when we consider the timeframe of what we're talking about and all the constraints

the engineers are trying to work around, it's pretty amazing. For starters, there's the need

to somehow manage the boost that the supercharger is producing so you don't blow the engine up at

low altitudes. There's the fuel that the engine will burn, which in some cases could be 87 octane

and in others could be well over 100 octane, and finally you have to package it all in the

relatively tight confines of a 1930s or 40s fighter plane. In order to balance all these issues,

a whole slew of options were bandied about and ultimately created to properly quote

altitude supercharged aircraft engines in the war. You had engines with single-stage superchargers

geared to the engine, a single-stage supercharger with a two-speed mechanical clutch and transmission,

a single-stage supercharger with a variable speed hydraulic clutch,

a two-stage supercharger with a mechanical clutch, a two-stage unit with a hydraulic

variable clutch, a two-stage after-cooled variable speed clutch unit, and turbo exhaust-driven

superchargers with an intercooler. These different methods were employed by different countries,

sometimes multiple countries using the same setup, and they were done to create the best

high altitude aircraft possible, which also performed well and reliably at lower altitudes

without overworking the engine and blowing it up. Did they all work? To a degree, yes,

and that's what we need to explore now. Before we do that, let's define a few terms you'll

be hearing in this video. First off is absolute ceiling. This is, as it sounds, the maximum

altitude an aircraft can operate in. Not many factors contribute to these things, but safe

it to say, when this altitude is reached, the aircraft can no longer climb higher.

Service ceiling is defined as the maximum altitude an aircraft can operate at with a given load,

with normal responses from its controls and still being able to climb.

The last one to talk about is critical altitude. This is where the plane can achieve its top speed,

a place where the engine is making big power, the air is thin, and the performance envelope of the

craft can be reached. It's also the area that, once breached, sees the engine making less and less

power. Of course, all these numbers vary with an airplane's design, not simply the horsepower of

the engine or how it's boosted. Like all things in mechanical design, everything is a compromise

to some degree or another, especially with regard to airplanes. Okay, now that we have that defined,

let's go back to our methods of supercharging. The single stage one speed centrifugal supercharger

is where most everyone starts in this journey of warp boost. In one notable case, it's where a

well-known engine stayed in its majority form, leaving it with a slightly lame reputation.

Yes, as much as I love the Allison V-1710 airplane engine, it was not all that well handled during

World War II. Created in the 1920s originally as an airship engine, it was well designed,

rugged, and had the mechanical bones to stand in the ring with anything else the world threw at it.

Unfortunately, it was let down by its use of a single stage supercharger that was never really

improved upon due to reasons we'll get into in a little while. At the start of the war,

the Allison was making comparable power to rival engines and was capable of sending aircraft to

rival levels of altitude. But as time went on, supercharging improved everywhere else,

but the Allison was largely left behind. This is why the Mustang, Spitfire, and other truly war

changing aircraft used the mighty Merlin V-12 and the Allison was relegated to lower altitude,

ground attack style planes. As the rest of the world invested in much of the technology we're

going to go over here in the following minutes, the US left the V-1710 with its less than stellar

single speed single stage supercharger. This prevented the engine from being much good above

15,000 feet. While it may have made 1500 horsepower, that number would fall off steeply the higher it

climbed. The Allison had a really strong bottom end of its physical construction, had great cylinder

heads, but it was also hampered by a crummy intake manifold design that we'll also revisit later.

Long story short, in the grand scheme of things, the world never got to see the Allison perform

at its peak because the choices that were made in its usage and development really did hamper it.

More on that later. Lastly, as the Allison powered airplanes are often used in lower

altitude applications, pilots had to be very careful not to over boost the engine. In American

airplanes, they were responsible for doing that themselves with various controls in the cockpit.

Of course, there were loads of opportunities for mistakes, but like this formerly confidential World

War II memo tells us, guys would also use the max power settings to simply make more speed.

Note that the author here is saying that with 66 inches of boost, the engine will likely be near

1800 horsepower at 3000 rpm. This is in 1942, a few revisions and a few years into the conflict.

This was highly discouraged behavior because it would inevitably lead to someone

melting down a perfectly good engine. But as they say, boys will be boys.

Next up, we move from the simple single speed single stage supercharger to a single stage

two speed supercharger. Effectively, we have the same unit as before, but with a clutch and a simple

gearbox to step the speed of the supercharger up when we get our airplane to higher altitudes.

Of course, the benefit to this system is that by spinning the supercharger faster and moving

more air, we should make more horsepower at higher altitudes. All this is well and good,

but the reality is that there are limits to these things, and one of them is the efficiency

of the supercharger themselves when it is in theoretical high gear. At this time in history,

meaning the 1930s and 40s, the relatively rudimentary impeller wheels in these superchargers would

start to have real trouble when the outer tip speed passed over 1300 feet per second or about

886 miles per hour. Not to mention that at some point the supercharger becomes so inefficient

that the heat generated by the discharged air to the engine can have a drastically negative effect

on detonation and other issues and simply cause a loss of power. So even this two speed system

has its drawbacks. Pilots would have to be conscious of their engine RPM when in high gears to not

overspeed the supercharger and cause any number of issues, the least of which was likely a loss in

power, the worst of which was a supercharger killing an impeller and sending its guts through the engine.

The two speed single stage system was largely an early war stop gap in many ways and wasn't

the favored method to make the most efficient and maximum power on a warplane. Interestingly,

the clutch to engage the second speed in this arrangement could be engaged in a couple of

different ways. Engaging the clutch manually had to be a very violent experience for both the airplane

and the pilot, but there was another way this worked as well. Some of them had an altitude control

unit that would automatically engage the second speed at around 7 to 9,000 feet when the aneroid

control kicked in. The power and performance curve in this style of two speed supercharger looks

almost like the teeth on a saw blade. There are radical jumps when the second stage was instantly

engaged. It was hard on equipment, but there are other ways and better ways. Now it was German

engineers who favored the next approach and in many respects did it the best. This is a single

stage supercharger with a variable speed hydraulic clutch. Now the major benefits of this system

should be kind of obvious. Removing the human element from the equation by using a hydraulic

clutch that would apply itself relative to the altitude of the aircraft meant that the engine

was in theory always seeing the optimal supercharger speed for where it was in relation to sea level.

It drastically smooths out the power curve and allows the pilot to concentrate on his job

of attacking the enemy, not worrying about trimming fuel, adjusting throttle position,

or having to study his altimeter in order to engage high speed gear for the supercharger at

the right time. The interroid controller did all of that for the pilot. Now the airplane itself does

not see a drastic fall off in power and then a spike when the blower speeds up. Now this hydraulic

engagement style of clutch doesn't necessarily improve the maximum output of an engine,

but like a dyno curve that gets improved when power is picked up through the rpm range before

getting to peak, this makes the plane a better performer in the middle range of its altitude

before reaching critical altitude where power and performance begin to drastically drop off.

The German DB engines had their superchargers mounted not in the rear of the mill like an

Allison or a Merlin, they were mounted on the side. They were run by bevel gears in a jack shaft

and the fluid coupling was installed on the jack shaft which would control the speed of the supercharger.

Now the mounting seems curious, but it does allow the supercharger to ingest the air

and throw it more smoothly toward the intake manifold. It eliminates one of the bends that

the other design has when the air has to turn sharply to get into the blower and then sharply

out of the blower to get to the intake manifold. Next up is the two-stage centrifugal supercharger.

This is where the Merlin engine outshone all others of its era. A brilliant mathematician named Sir

Stanley Hooker began working at Rolls Royce in the late 1930s. He was an expert in studying what

we'd now call fluid dynamics or theoretical aerodynamics and this guy could basically see air.

He was an unbelievably brilliant man and he was able to look at, having never seen an aeronautical

engine or a centrifugal supercharger, what needed to be fixed to make them better. He was tasked

with making what amounts to a glorified air pump work more efficiently. Now after improving the

company's single-stage blowers drastically enough to raise output by 30% of the Merlin,

he had the concept of mounting a pair of superchargers together, one feeding the next.

Taking a supercharger off of one of the company's larger engines and combining it

with the Merlin's unit, he created the finest blower of the war and it allowed the company

to effectively double the output of the engine by the time the conflict had ended. Hooker knew

the bones of the Merlin were sound. It didn't need to be expanded in size or drastically reworked,

it just had to do its job more efficiently. And the two-stage, two-speed supercharger was the

ticket to doing just that. With this improvement, the engine was capable of much more altitude.

Its compact packaging meant that it would fit into existing applications with little rework

and its performance was improved in breathtaking fashion. As we can see here, the critical altitude

was stretched out by a bunch and while it gave up some performance at lower altitude,

it was of little concern. Flying higher and faster was always the goal. These units allowed

the smaller supercharger to work at lower altitudes without overboosting the engine,

but the clutch would be engaged at higher altitudes to deliver the extra horsepower punch

needed to out-climb the enemy and attack from above. Combining all the above with the fluid

coupling we spoke about earlier has an even more positive effect because this allows for the same

benefits as it did with the single-stage clutch, but in an even more dramatic fashion.

There was no dramatic drop in performance before an aircraft got to the proper altitude to engage

the second blower because it would all happen smoothly and perfectly matching the conditions

the plane was in. Effectively, this was getting close to the best of both worlds.

The next evolution was the use of the two-stage supercharger with the fluid coupling with the

addition of an aftercooler. This again was a large performance enhancement because of the job

the aftercooler did. Remember that we mentioned fuel types and characteristics earlier in this

discussion. As gearheads know, the higher the octane rating of fuel, the more resistant it is

to detonation or spontaneous ignition happening before it should inside a cylinder. An aftercooler

was a massive improvement in power making at altitude because it was placed behind the second

stage of the blower. It cooled the air charge as it entered the engine. Now as air is compressed,

of course it gets hot. Hot air entering a cylinder can lead to detonation and detonation can lead to

engines failing, especially when they are being worked hard, very high up in the sky. Higher octane

fuels like the 100 plus octane gasoline being developed in the United States during the war

helped to stave off detonation, but cooling the intake charge made this even better. By cooling

the charge, engines could run higher compression ratios. And again as gearheads know, high compression

engines plus crummy fuel can lead to detonation, which is why you don't put 87 octane in your

high performance high compression engine, but if you're able to manage that with a better fuel,

a cooler intake charge and more compression, you're going to make a lot more power.

As we can see here, there's a drastic improvement in the critical altitude and power throughout

the whole operating range of the aircraft. The downside is you have to find a place for

the after cooler and adding stuff to a plane can make it heavier and less aerodynamic.

Engineers got creative with their placement of the after coolers,

preventing a loss of aerodynamic performance and allowing for the additional horsepower to be used

rather than wasted trying to push the airplane through the sky.

Lastly and perhaps most interestingly, we get to turbo charging, or as it was known at the time,

turbo supercharging. If you remember at the beginning of this exercise, I mentioned that

one country seemed to make a large scale mistake when it came to their chosen method of World War

II era supercharging. That country? The United States. The United States decided very early

on that turbocharging was going to be their root of two stage boost making in aircraft.

They did this for good reason. A turbocharger is the most efficient method of making boost

and it did things that the gear driven centrifugals could not dream of doing.

As the turbocharger doesn't use the engine's power to be turned, but rather the heat energy

and the exhaust to do its work, it's a vastly superior way to make power versus a mechanically

driven blower. In fact, the turbo supercharge combination has the most amazing power curve

of all we have seen so far. It raises the critical altitude many thousands of feet

while producing virtually the exact same amount of horsepower at that height as the engine does

at sea level. So what was the mistake? Packaging. Sanford Moss of the General Electric Company

was the man who developed these systems for the military at the time when the United States

committed to this method, all work on improving gear driven centrifugal superchargers basically

came to a halt. The Allison was hampered severely by this in almost all applications outside of

a handful of legendary ones, most famously the P38 Lightning. A large plane, there was plenty of

room for all the associated plumbing, the intercooler and the monstrous turbochargers.

In today's world, turbos fit comfortably, most of the time, under the hood of an automobile.

In the 1940s and on engines this size, they required multiple people to actually lift them

and move them. Turbochargers in their entire systems were too hopelessly massive to fit

in any aircraft like the P51, the Spitfire, really anything of that class. And there were other

problems as well. In the European theater at high altitudes and freezing air, the turbos and

associated parts would fail and cause severe mechanical issues. When the P38 was brought to

warmer climates like the South Pacific, they really did stand out in combat. Their power,

altitude capability and performance were awesome. But unfortunately, this technology was so far

ahead of its time in terms of pure bulk, it wasn't adopted widely and was very much more

problematic than the other more proven methods. Had the Allison gotten the Sir Stanley Hooker

treatment in volume and in different applications, its reputation and legacy would be vastly different.

Turbos were successful on radial engines as well, most famously the large by huge P47,

which was basically an engine with a plane built around it. Again, breathtaking and

awe-inspiring performance for the time, but the bulk they brought was just too much to overcome in

so many other places. In turbo supercharged applications, an intercooler was used. Note

the difference here. The aftercooler we spoke of before would cool the charge after the second

stage of the supercharger as the air was heading into the engine. In these planes, the charge would

be cooled heading out of the turbo and then into the gear driven blower attached to the engine.

Hence the inter part of the intercooler word, it was happening during the middle of the intake

process. In many ways, intercooling is not as efficient as aftercooling because as the air

went through the gear driven supercharger, it would get heated up and compressed on the way into the

engine, but it still worked very, very well. When it worked. In period literature, it was claimed

that the turbo supercharger could deliver sea level power up to 30,000 feet. Unfortunately,

the size and heft of the whole package was really only suitable in large bombers or

the large fighters that would fly high in the sky to attack the enemy's bombers,

but for the live fighter planes like we mentioned, it just wasn't an option.

Consider this the first step into what will be a much deeper walk into the world of World War

II supercharge liquid cooled aircraft engines. As I mentioned, there is an incredible depth

to this subject and it is one that employed the best mechanical engineers in all of the

warring countries to refine and make better. Perhaps much of this is common knowledge to

historic airplane buffs, but I am talking gear heads here, some of which may not have known

the extent to which boost, mechanical supercharging and turbo supercharging played a major role in

the outcome of the air war that gripped the entire world in the late 1930s into the middle 1940s.

Next time you revisit this topic, we will take a look at the fascinating life and times of

Sir Stanley Hooker, the man who could see air, a brilliant mathematician who had perhaps the

most dramatic impact of any single man on the mechanical fronts of World War II.

I am Brian Loans, thanks for watching. Like and subscribe for more historical,

mechanical and altitude busting content.

War Boost: An Introductory Lesson In WWII Aircraft Engine Supercharging

About this episode

horsepower

octane

piston engines

liquid cooled engines

centrifugal superchargers

boost

supercharging

Allison V-1710

Ford Mustang

gearbox

efficiency

detonation

Rolls Royce

fluid dynamics

output

blowers

Merlin

two-stage supercharger

aftercooler

turbocharger

P-38 Lightning

intercooler

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