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October 28

Fuel efficiency (auto milage)

All else being equal, does the standard fuel-using car travel further on higher octaine gas (petrol) than on “regular” gas?DOR (HK) (talk) 19:31, 28 October 2019 (UTC)[reply]

As long as you are using the minimum-required octane for the vehicle, no, though this is a common misconception. Octane rating: higher-octane fuel can be compressed more without autoignition; engine knocking occurs when the fuel detonates prematurely on its own (due to being compressed and heated), rather than by the spark plug. High-performance Otto cycle engines typically have higher compression ratios, which means the fuel needs to withstand higher pressures and temperatures without autoigniting, meaning a higher octane number. Putting fuel with a higher-than-required octane number in a typical engine does nothing other than make the engine burn more expensive fuel; the engine is incapable of compressing the fuel more, which is what is necessary to achieve better performance. (I'm sure there are some exotic engines with adjustable compression ratios, but of course you specified a "standard" vehicle.)

Potential article improvement: anyone think octane number or another article should state this explicitly? --47.146.63.87 (talk) 00:46, 29 October 2019 (UTC)[reply]

Also you need more energy to ignite it, which is drawn from the battery, whose power is in turn refilled via dynamo by burning fuel. So you'll actually get slightly worse mileage. You need to have good enough gas to avoid unpredictable autoignition since the power it creates doesn't go fully into turning the crankshaft but is wasted on the engine walls (and causes long term damage), but once you've minimized the occurence of that there are no benefits on mileage for using better and better gas. 93.136.155.134 (talk) 01:32, 29 October 2019 (UTC)[reply]

Computers that adjust compression via timing are not exotic. For example, the very common Dodge 5.7 V8 has variable timing (and it shifts into 4 cylinder mode when all 8 aren't needed). 2600:1004:B022:5255:7A88:5987:105B:29D9 (talk) 09:44, 29 October 2019 (UTC)[reply]

Hmm, the Dodge 5.7L Hemi? That does have variable displacement, but does adjusting the ignition timing actually change the compression ratio? It looks like you need to actually change the engine geometry to do that. I found the article on such engines: variable compression ratio. --47.146.63.87 (talk) 20:13, 29 October 2019 (UTC)[reply]

If you fire when not at full compression, you effectively change compression. — Preceding unsigned comment added by 12.229.60.66 (talk) 23:35, 29 October 2019 (UTC)[reply]

To be painfully exact... Cylinder compression is based on the geometry. That is one factor of interest in fuel efficiency. Another is cylinder pressure, which based on geometry and the amount of fuel-air injected into the cylinder. The two are often mixed together in conversations. Another is firing time. If you fire early or late, you fire when pressure is not optimum. Further, firing early will likely cause issues with the piston because it is being pushed up by the shaft below and being pushed down by the explosion above. Firing late fires when pressure is lower. This would be equivalent to having a shorter cylinder - which is a different geometry. I personally do not know of any engine that purposely fires late to improve efficiency - but that doesn't mean it doesn't exist. Yet another factor is the spark itself. Too much fuel is injected because the optimal fuel mixture is hard to ignite. A dual-spark system allows you to use less fuel. Some engines have a separate, very small spark chamber that has a high fuel mixture and then very low fuel mix in the large main chamber. The flame from the prespark ignites the main chamber. But, the question is about octane and related fuel efficiency. High octane fuel, by itself, isn't more or less fuel efficient. What high octane fuel allows for is better performance. Simply put, you don't have to push down on the gas pedal as far, so you end up using less fuel. The end goal is to put less gas into the engine. Turning off cylinders is one effective way of doing that. Using one of the many forms of pre-ignition or pre-spark is another. Even a dual-spark plug system helps. Most of all, it is up to the driver. If you are cramming down on the gas pedal, you are using more fuel than you need to use. It isn't the engine's fault. 135.84.167.41 (talk) 14:19, 1 November 2019 (UTC)[reply]

Many thanks for useful and on-topic answers. DOR (HK) (talk) 16:39, 29 October 2019 (UTC)[reply]