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padgett 11-03-2012 12:31 PM

Gas vs Diesel
Interesting chart at Gasoline gallon equivalent gives the BTU/gallon of different fuels. Note that the octane of gasoline makes no difference in the energy, is just how fast it burns. However diesel has almost 14% more than gas.

bill_de 11-03-2012 12:36 PM

Re: Gas vs Diesel
The only reason many people think otherwise is that gas companies, for years, advertised that your car will run better, faster, or get better mpg if you buy their premium product.

padgett 11-03-2012 06:04 PM

Re: Gas vs Diesel
The issue with gasoline and octane is related to combustion chamber size and compression ratio (n/a, if boosted everything changes). If you read all of the article you also see that a gasoline engine is less efficient than a diesel.

Are a lot of reasons why (best explination I have ever seen is in Sir Harry's book) but basically a traditional gas engine relies on the flame propagation rate (how fast the flame front spreads from the plug to the edge of the chamber). Key is for the charge to burn completely before a second flame front can start. If you get a second, the collision between the two is where you get ping or knock, the chamber pressure spikes and drops off early, generating heat but no power. Not good.

Higher octane means it burns slower and can take a higher chamber pressure/temperature without generating another flame front.

I keep repeating flame front and flame propagation because to understand a gas engine requiires that you think of it as a progression and not an explosion, without that all of the other kludges might as well be magic.

Direct inject is going to change that. Diesels have used it for years (they rely on compression ignition or knock/ping/detonation to work). With DI you just need to be concerned about the pressure rise rate and not so much flame progression. Compression ratios can rise (have seen mention of 13-15:1 for DI gas engines) and so can efficiency/MPG.

Waving a tie soaked with TEL over the intake will have no effect. There are a lot of benefits but engines will need to be stronger (more diesel like) particularly in the head sealing and the valves. There will also be a need to run chambers cooler to control things like NOx but that is a secondary issue.

Will say things are going to get interesting.

BTW I see the Ram 1500 is being advertised at 25 mpg with the 8 speed. Have also seem 305 hp mentioned. Anyone know if they changed the cams to make them a bit peakier than the 2012 engine ? Would really like to see a torque curve.

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