What is greener: premium or regular gas?

You can make less from a barrel presumably because there's more stuff in it.

Premium should be better, because it has more energy in a smaller volume and theoretically the thermodynamic efficiency of the engines is greater.

 

Both comments are incorrect. A unit of premium requires the same amount of feed stock as a unit of regular. Likewise a unit of premium does not contain more chemical energy.

The only difference is the resistance to spontaneous ignition, which is called "octane" in the fuel world. The blend for premium contains petroleum products that are harder to ignite, or additives that produce a higher octane rating. This adds cost.

Premium fuel has essentially the same specific energy as regular. Depending on how the higher octane rating is obtained, premium fuel may actually have lower specific energy than regular. This is true in the case of ethanol, when it is used as an octane booster.

The advantage of premium fuel comes from being able to use higher compression in engines. Higher compression allows for more efficiency. The advantage is in engine efficiency, not inherent energy in the fuel.

Diesel engines take this another step, and are the logical end point to this discussion about premium gas. Diesel has a very high octane rating, is cheaper to produce, and allows for higher efficiency engines.

Tom

 

Premium can be produced from cracking the hydrocarbons in regular gasoline. This costs the refinery more money and a little more energy. Its not more stuff, its the same amount of stuff but reconfigured. Premium only burns better in an engine designed to burn it

 

Tom no...diesel is very low octane but high cetane no. This is because diesel has the molecules which ignite by themselves under compression. High octane molecules are those which resist auto ignition and wait to get the spark from the spark plug.

 

I knew that premium allowed for higher compression and therefore higher thermodynamic efficiency. And I knew that you needed a higher compression engine to benefit from it. (Which leads to the question, how on earth was I thinking that I it had more energy? I guess my brain mixed it up with the E10/E0 problem.). But, that you need a better engine doesn't really factor into the greenness.

However, I'd actually assumed that the cracking worked the other way, in that you break down the chemicals to make the combustion easier rather than harder and that premium's cost came from more selective distillation. (Edit: again, duh, CNG: small molecules, allows very high compression; yes burns nicely, but that's not the point, it's not igniting on compression that's the issue).

Anyway, I'd still say that the potentially higher efficiency provided by premium makes it greener.

 

Premium is a mix of higher octane materials and is more expensive to make. Ethanol is high octane and so is an aromatic such as toluene. So it does not necessarily contain more energy but often is richer in the aromatics so in general probably higher density but that is not mandatory. As a pratical matter there are no cars designed to get better MPG based on fuel composition so I would say premium is slightly less green due to extra work to make it. Let's say you could design a car to run on all toluene maybe you could get better MPG but there is no market for that.

 

To me, something is green if it increases the potential for sustainability and decrease environmental damage.

A good example is turbocharging. Originally it was used to increase power, now it's being used to decrease displacement. The potential, however, was always there.

 

There are heavy components in the crude oil, that are cracked into other hydrocarbons. Premium requires a more expensive processes in a catylitic or steam cracker, think of it more of a different crack of the oil instead of a crack of gasoline I mispoke a little before and that can be confusing. In the old days they only needed to distill and blend gasoline, now to get a higher fraction other portions of the oil are cracked. I agree that it is greener to burn premium in an engine designed for it.

Ford has the bob cat that can take advantage of e85s properties as well as work with normal gasoline
Sneak Peek! Ford's "Bobcat" Dual Fuel Engine - PickupTrucks.com News

 

Yes, my error, current diesel fuel is not high octane, but that doesn't change my point about high compression efficiency. The diesel engine is the logical endpoint in this discussion of compression.

Actually diesel is low octane because it doesn't need to be high octane. A diesel engine can burn just about anything, although they are optimized for what we call diesel oil. The original design was intended to burn coal, but there was no reliable method for injection.

Tom

 

In the old days, yes less processing but lead TEL tetra ethyl lead was used to boost octane, an eco-bad actor but one helluva nifty octane booster for ICE fuels.

Yes refineries do have cracking to make gasoline from heavy oil.
But usually people talk about the platinum reforming process as the octane maker. The platinum catalyst makes aromatics which are higher octane, and higher energy content. Even though Premium does not need to have higher energy content in theory, one would think it generally does have more energy content.

This suggests blending Premium and E10 could tend to cancel out the ethanol negative MPG impact, but as far as I know no one has data like that.

 

Definitely not asking to go back to the bad old days of TE lead or MBTE. I was just suggesting much more of the hydrocarbons in oil are being transformed to be used as gasoline now.

I was using cracking generically. I really have no idea how much of the octane boosters are constructed from breaking longer chains or assembling shorter chains. Premium gas has essentially the same energy content though. It does take more energy to produce the components than the feed stocks.

If your mid grade gas is burned in an engine designed for it, the greater efficiency should cancel out the lower energy content.

 

Auto industry engine designers apparently decided 87 octane was high enough for most cases. If the engine designers thought they needed higher octane (eg; for CAFE MPG increases) I don't think they would hessitate to require the refineries to make more Premium, since it can be done obviously.

 

This is the key to the whole discussion. Market pressures have converged to standardize on 87 octane. If higher were cheaper overall, we would see a trend in that direction, but we don't.

Tom

 

Interesting...

 

...since we had this discussion, believe someone (Drinnovation) may have pointed out in another thread out that Chevy Cruze gets the good EPA mileage on Premium, but can run Regular. In that case perhaps Premium is greener.