Should we bring back high compression engines, to save fuel? Definetly more efficient -how much?

Question

In the 70s, EPA rules caused a decrease in compression ratios, for 2 reasons:

1. eliminate lead as "octane improver"

2. reduce nitrogen oxides emissions

BUT:

1. Ethanol as an octane enhancement? - some dragsters use pure ethanol @ 15:1 ratio

-also: more octanes only needed @ wide open throttle - which allows cylinder to be filled more completely. Did car makers switch to 4bbl carbs, to maintain power, by using more air/fuel, less efficiently?

2. If speed limits were reduced, smaller less powerful cars would gain practicality. combined with more efficient combustion, perhaps overall NOX output wouldn't gain much. slower traffic would also get more people into bicycling.

? efficient high compression engines, as reason for cars used in other countries getting better mileage

- I think high compression is one reason diesels are more efficient.

- high power/weight applications use high comp. - motrocycles, airplanes

- high ratio allows more expansion of hot gases

Answer 1

Blah, blah, blah.

"I think high compression is one reason diesels are more efficient" - Wrong, diesels are high compression because it requires more compression to induce diesel fuel to spontaneously combust from a glow plug, as opposed to a gasoline/ethanol engine that relies on a properly timed spark plug to create combustion.  Diesels burn a much lower percentage of the air/fuel mix, and have a higher average output of hydrocarbons compared to gasoline or ethanol.

"high ratio allows more expansion of hot gases" - Wrong.  Higher compression ratios allow a larger fuel/air mixture to be compressed and therefore combust, compared to the volume of fuel and air that could be completely burned in a lower compression environment.

What cars from other countries are using higher compression engines and getting better mileage as a result?  Please give more information on these cars and what countries they are being used in.

"slower traffic would also get more people into bicycling" - Wrong.  The average commuter travels 23 miles per day, I really don't think dropping the speed limit is going to cause a major shift in our transportation habits.

I'm getting bored with you.  I'll stop now.

Answer 2

The EPA also changed the specs on ignition timing. It is now 10 deg before top dead center ,and it was 13.. This alone cost us about 17% ,and did reduce the NO2.