Honda Patents Wacky, Radial, Variable Compression Engine

Sebastien Bell
by Sebastien Bell

Honda might be looking back to early aeronautics in its latest bid to make variable compression engines simpler and sturdier.

The engine is designed to take advantage of the simple crank design of radial engines to make a robust and widely adjustable variable compression engine. By replacing one of the pistons in the engine with a moving lever, the compression can be easily changed, leading to more power or efficiency, depending on what is desired.

The engine works a lot like Infiniti’s VC Turbo, but the different cylinder layout means fewer moving parts, which the patent argues will make it more reliable.

Radial Engines

The radial engine has been described as a starfish doing a striptease because the engine surrounds its crankshaft in cylinders. That means that instead of having, say, five-cylinder arranged in a straight line above a long crankshaft, all five cylinders are arranged in a star pattern surrounding a very short crankshaft.

This is useful in airplanes because you can put a bunch of cylinders right at the nose of a plane and the air cools them all equally. In cars, it’s less useful, though, because it’s a big lump of engine to design around.

Back in April of 1969, Popular Mechanics wrote about a super secret Pontiac test car called the X-4 that used a radial engine to make 80 hp. That engine was slotted inside a 47-inch tall commuter car. Although the car was already quite small, Pontiac could only fit two sits in it because the giant 80-hp engine in the middle of it took up so much space.

Honda, though, has shown a couple of engine layouts — and that’s what has us particularly interested this patent — that shows how this could actually work in a car.

The drawings show the engine aligned transversely in a van and longitudinally in a car. The drawings also lean pretty heavily on the narrow-angle of the cylinder Vs. So unlike the X4, the cylinders are arranged like two Vs, rather than an X. >< not x.

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That could be important not just for packaging, but for repackaging. There’s only a brief mention of it, and the application has been translated from Japanese — so there’s the potential for miscommunications here — but it appears that Honda has designed this so that it could also run as a narrow-angle V-twin, which would be useful for motorcycles and even potentially hybrids.

For both of these ideas to work, though, the cylinders would have to be arranged in pairs, which raises some timing issues. Radial engines are traditionally made in odd numbers because the even numbers would to an unsmooth (four-stroke) engine.

However, while two drawings of cars buried deep in a patent application does not confirmation of intent make, it’s worth bearing in mind that variable compression has been a focus of automakers lately.

Why Vary Compression?

Variable compression is the latest engineering buzzword since Infiniti came out with its VC Turbo last year. That’s because the engine, according to Infiniti, has the power of a big displacement engine with the efficiency of a tiny engine.

And although they’re the first to make it to market with a VC engine, they weren’t the first to think of it.

At the turn of the millennium, SAAB was working on a VC engine that actually makes the concept of variable compression a little easier to understand.

The idea behind VC cars is, put simply, to actively change the size of the cylinder. SAAB achieved this by putting the cylinder head on hinges. That means that you can play with the amount of space where explosions can happen, which, in turn, affects how much power is generated. More space means bigger explosions. The downside, though, is that you’re also using more fuel.

If you have variable compression rates, though, you can essentially have a small, efficient engine when you need it and a big powerful engine when you want it.

Infiniti does this by moving where the piston extends to lower in the cylinder. More accurately, they twist the bottom of the con rod to change where it sits on the crankshaft, effectively increasing displacement.

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Since the Infiniti engine is an inline 4, though, each piston needs its own twisting mechanism, all of which is heavy and complicated.

Honda’s system works similarly, but is simpler, because you only need one twisting mechanism for four pistons. Actually, you could more than that. Radial engines frequently have five, seven, and nine cylinders, but we’ll keep talking about four cylinders since that’s what the images show.

Radial engines* arrange their cylinders around a single common point. So all four pistons, in this case, are attached to one ring that fits onto the crankshaft. What Honda wants to do is attach a lever to that ring, which would twist the connecting rods and alter the size of the combustion chamber.

Theoretically, Honda could do what Infiniti is doing with four twisting mechanisms, with just one. The lever, explains the patent, can also be moved however much or little is needed.

Rather than just being on or off, choosing between two compression ratios, the lever could move just a little to one side or the other, making it more precise.

But is it Worth It?

Unfortunately, the first production VC engine hasn’t exactly set the world on fire. While the improvements have been noticeable — the Altima gets about 3 mpg better than its competition — that might not be enough to warrant the changes needed to introduce a radial engine into the lineup.

The advantage of Infiniti’s more complicated set up is that the engine (as far as space within the engine bay is concerned) is pretty much the same as any other engine in Infiniti’s lineup. That means that the cars don’t have to change meaningful to fit the engine in them.

A radial engine would seriously affect packaging and it would be extremely difficult to fit this engine into, say, a Civic (it would be useful to show you a picture of the Pontiac X-4 here, but since we don’t own the rights to that pictures, you’ve got to click this link). That would make production very expensive.

To put it in perspective, radial engines were too big for tanks. In the Second World War, U.S. tank makers thought that the engines might be useful to put into tanks because of their high power to weight ratio, but found that the design made tanks tall and susceptible to attack.

So even in a vehicle famed for its size, the radial engine is difficult to package.

Even if you argue that the Honda engine can be smaller because it can do more; even if, despite the layout, it was roughly the size of a regular inline four, the position of the crankshaft — right in the middle of the engine — means that you can’t just slot it in where an I4 or a V6 would go. So Honda would probably have to design a whole separate car around this engine (if that is, indeed, what they have planned for it).

Some of the traditional arguments against radial engines probably aren’t as big a deal anymore, though.

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Radial engines are often accused of being hard to work on. It’s just hard to reach all of the spark plugs. That might be less of a concern today, though, than it was a few years ago. Fewer and fewer people care to do their own work and engines are ever more complicated.

And despite it potentially requiring a whole chassis, Honda has shown that there are some ways of placing the engine that could work reasonably well.

Whether or not efficiency gains of 27% to 30% — that’s the amount that Infiniti says its VC system improves efficiency — are worth all of the packaging difficulties and production costs is a question only Honda can answer. But the engine described in the patent looks really neat, so we hope they say yes. We can’t help but wonder, though, if the electric car will cause this engine to suffer the same ignoble fate as the Pontiac X-4.

[Radial engine gif by Stoianovici [ CC BY-SA 1.0], via Wikimedia Commons]

[SAAB engine photo by Reedhawk – Own work, CC BY-SA 4.0, Link]

Sebastien Bell
Sebastien Bell

Sebastien is a roving reporter who covers Euros, domestics, and all things enthusiast. He has been writing about the automotive industry for four years and obsessed with it his whole life. He studied English at the Wilfrid Laurier University. Sebastien also edits for AutoGuide's sister sites VW Vortex, Fourtitude, Swedespeed, GM Inside News, All Ford Mustangs, and more.

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