Honda Patents Diff'rent Strokes for Diff'rent...Cylinders

Since the introduction of Cadillac's V-8-6-4 in 1981, variable displacement has become another tool in the automaker's box as a way to improve mileage while retaining power. While the ill-fated Cadillac system was not quite ready for prime time, the idea was sound. It takes a lot of power to accelerate a car but not much to keep it moving, especially on our largely flat highways. Anything less than about 100 hp is going to be far too slow to be enjoyable for just about anybody in a modern car, but it can take less than 10 hp to keep it going at highway speeds. It is very difficult to make an engine that is powerful enough to make over 100 hp, but then is still very efficient at 10 hp. Losses from friction, the spinning mass of the pistons, rods, and crank, losses from the vacuum trying to pull air into the engine, it all adds up. The idea behind variable displacement engines is that instead of injecting a tiny amount of fuel into every cylinder you just turn off the fuel to some of the cylinders. You still have the friction, but you don't have to waste or inefficiently use the fuel required to keep the cylinder firing. You can only reduce the amount of fuel injected so much before it won't ignite, and that's both wasteful and bad for emissions. Manufacturers have discovered that if you can keep the valves closed on the "dead" cylinders, the air in the cylinder acts like a spring, so the force used to compress the air is almost entirely regained when the air expands. It even gets rid of the losses to the vacuum sucking air into the engine. You can effectively turn your 3.0L V6 into a 1.5L four, whenever the computer says it’s OK to do so. When you need the power, it’s there at the touch of the pedal and a click of some solenoids. It's a great solution to a common problem.

It took modern electronics and variable valve timing technology to make it a reality, but variable displacement (also known as "multiple displacement") is now featured in a large number of V8 engines, some V6's, and a handful of four cylinder engines. Even Aston Martin's new V12 will have it. Most systems work by simply shutting off the injector for the chosen cylinder, then cycling a solenoid to keep the valves closed. Some systems, like GM's, shut off the same cylinder every time, and some, like Honda's VCM, change up which ones are shut off to prevent premature wear. The systems can introduce some imbalance, especially on six-cylinder engines,  but manufactures are now using engine mounts that can actively change to cancel the imbalance, and use active noise cancellation in the stereo to get rid of the noise.

There are some limits to the system. One of which is that you’re limited to how many displacement choices an engine can have. After all, if you have six cylinders, they’re all the same size, right? So you can have your 3.0L six be 500cc, 1000cc, 1500cc, 2000cc, 2500cc, or 3000cc. Sure that’s a lot of choices, but they’re still big jumps. On a four cylinder, you only get four choices, and each one of them takes away 25% of your displacement. That’s a big jump between choices as well.  Honda engineers asked the question do all cylinders really have to have the same displacement? Well according to a patent they filed in 2014, that was published earlier this year, the answer is no. Cylinders do not have to be the same size. Honda accomplishes the different displacements not with a different bore diameter for each, but with a different stroke length. Each piston would have the same top dead centre, but would not all come back down into the cylinder the same distance. Picture a four cylinder with pistons that were 300cc, 425cc, 625cc, and 650cc. That can give you a much smaller total (300cc), or a much larger (1,700cc) total, with more choice in between That way you can customize the engine to give you smaller gaps, more choice, and more flexibility. A four cylinder engine with four different piston sizes would have 15 different displacement choices.

Does this sound a little crazy? Yes, but Honda has never shirked away from engine innovation. Like how in 1979 they made a motorcycle engine with oval pistons so they could put eight valves in every cylinder. Or the Compound Vortex Controlled Combustion (CVCC) engines that could meet ‘70s emissions regulations with no catalytic converter.  And lets not forget VTEC variable valve timing.

Would the engine computer use all of the possible displacements? Probably not, but the computer will know the best ones to use. For example, gasoline engines are generally more efficient at wider throttle openings as this reduces the losses from sucking air in. The computer knows this, and can then adjust the throttle by wire and variable displacement to use the smallest displacement and widest throttle opening to maximize economy. If you ask for just a little bit more power, lets say for a slight incline, the engine doesn’t have to jump up 25% in displacement. It could jump up just 10%, or 15%.The biggest downside would seem to be the vibration from uneven firing, but Honda is already at the forefront of that with active sound cancellation and engine mounts. That saves fuel, and saves money. Sounds good to me.