Variable compression engines

I just read an article in one of my automobile mags that describes Honda’s now variable compression engine. IMHO it’s a stunningly simple approach, and I can definitely see its advantages.

I have to point out that even with the diagrams the operation took some studying to understand. The narrative never mentions that the cylinder is not in line with the rotating axis of the crankshaft, and it took my realizing that to understand how the system works. No notes on the diagrams point that out either. Once I realized that the cylinders were askew from the crankshaft axis, how the system worked suddenly appeared obvious.

The magazine is Automobile. I was going to photograph and post the diagrams, but concerns about copyright violation stopped me. The article is worth reading, however.

Not sure what you mean. The axis defining the crankshaft rotation is 90 degrees from the cylinders, right?

Yeah I saw that article. I think I was coming to the conclusion that when all these features come out, you’d better just plan to trade before the warranty is up. At least the first batch anyway. I’m no engineer though so maybe its a carefree design.

Honda? I know Nissan is making one now, selling it in an Infiniti model.

Sincere thanks for the videos.
The sketches in the magazine showed the crankshaft axis as being not in line with the cylinders when looking down the rotating axis of the crankshaft. I could not determine from those photos how the system would have worked had they not been. The video doesn’t make that point clear, being from a mostly side angle, but it does clearly show the operation.

I wonder if they’ll become ubiquitous or if EVs will take over the market first.

Considering the investment in ICE power plants and the lack of maturity in EV battery design and remote charging stations, I think we will see a lot of vehicles with variable compression. IMO, the longest EV ranges need to about double with a lot of fast charging options before ICE availability drops much. Only the Teslas and the Bolt are reasonable choices for long drives, and I don’t think there are half hour or less charging options for the Bolt yet. If I had a Bolt and visited th BIL, it would take almost a full charge and then I’d have to recharge while there. Doing it on 120AC would mean I couldn’t use the car while there for a weekend, and he doesn’t have 240AC available. I’d have to wait a few hours at 240V, and the practical offshoot is that’s I would need two cars to recharge one; leave it and return later when fully charged at the pay site.

What advantage does variable compression ratio offer that super charging doesn’t?

Being able to optimize use of higher compression ratios in certain conditions maximizes engine efficiency, something a supercharger doesn’t do.

That’s a good question!

I don’t know, so just kicking it around…Both increase cylinder pressure. Both can be adjusted electronically with boost controls and the actuation for the variable CR.

Maybe it gives the ability to increase cylinder pressure at low RPM, lower than high levels of boost are developed. The CR could go high enough to compression ignite. But it seems like variable valve lift and timing could perform similar feats shifting from Otto cycle to Atkinson cycle.

A normal engine’s compression ratio is a compromise between efficiency (higher CR is better) and fuel octane. Engines will knock at different conditions depending on load, so the CR has to be set to handle all conditions. Variable CR lets the engine use higher CR (better MPGs) when conditions allow, dropping CR when required to prevent knocking. The Inifiti engine also is turbocharged, so all those other adjustments you mention will also be made.

primolzen’s comment, Too bad it’s from Nissan, and will be a heaping pile of mangled garbage within the first 5K miles.

------Too complicated = too much chance for catastrophic failure------

seemed fitting


I share Rod Knox’s concerns

I wouldn’t be surprised if Nissan is first to release this technology . . . yet Toyota or Honda are the ones who eventually perfect it and make it reliable

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Stick around and we’ll see if it works or not.

The article in Automobile magazine was about the Honda engine. According to the article it’s already planned for some of their models.

Variable compression is definitely an interesting idea. Seems like it would offer up some mpg/accel/emission advantages. On the downside, extra bearings and mechanisms all to fail, plus the engine is probably a little larger so may be difficult to fit in smaller cars.

Along the lines of new engine design ideas, I read something about Toyota engines now having a more sophisticated variable valve timing design. With dual VVT, both the intake and exhaust valve timing change (as opposed to single VVT where only the intake timing changes) and they even have one called Valv-o-tronic or something like that where the distance the valve is pushed open changes under computer control.

To me, variable compression looks like more complication than it’s worth, but maybe we’ll find out.
I had once thought maybe Ducati’s desmodromic (no valve springs) idea would have caught on big by now.
The Qamfree (no cams) idea is also intriguing.

I can find nothing about Honda’s variable compression ratio engine anywhere. All I could find was reference to a Honda patent for variable displacement.

It seems to me that it’s all about gas mileage improvements for the car companies. If it can provide more average power, they can achieve similar performance from a smaller engine, and more efficient fuel use.

I went back to the article and you’re correct. It was about an Infinity variable combustion engine. My bad.