Quote:
Originally Posted by Ecky
As I understand it, there are two good reasons we see 3 cylinder 1.0's and 1.2's rather than 4 cylinder, and I can speculate on a few more.
1) 3 cylinder and 5 cylinder engines have some inherent efficiency advantages due to the timing of the exhaust pulses over the more balanced 4 and 6 cylinder engines.
2) Volume of a cylinder goes up cublicly while surface area goes up by the square as you increase cylinder size. For this reason, a 1.0L 3 cylinder has less frictional surface and less surface through which to lose heat and therefore thermodynamic efficiency. A 1.0L 2 cylinder or even 1 cylinder would be even better in this respect, but it would probably rattle your teeth out.
Speculation on my part, but due to the flame speed of gasoline combustion, there will be a (maybe more than one?) Goldilocks zone for cylinder bore and stroke at a given RPM. If a cylinder's stroke gets shorter, you need to compensate by increasing the RPM to keep piston speed in that zone or you'll lose efficiency, and (if I remember correctly) friction goes up by the square of RPM, which you'd really want to avoid.
This is not something I've read anywhere, just something I've put together myself after thinking about it a good deal. Someone please correct me if I'm wrong.
|
There's also the question of how much combustion chamber real estate you can set aside for direct injection, which has to share that space with four valves and a spark-plug.
I recall Hyundai-Kia stating there were issues with putting direct injection on something smaller than a four-cylinder 1.6. But that was years ago... and we've come a long way since then... Obviously, it can be done and has been done, as we're seeing a lot of 1.5 liter four-cylinder DI engines... but other manufacturers are going for 1.5 three-pots to give themselves more wiggle room in cylinder head development.