OK, so a gasoline engine works by compressing a fuel/air mixture, then igniting it with a spark. If the compressed mixture gets too hot, it will ignite by itself, which is bad. We use higher-octane fuel to prevent this from happening.
A diesel engine works by just compressing air, then injecting fuel. The fuel ignites by itself due to the temperature of the compressed air, which is good.
So would it be possible to design an engine that compresses air, then injects gasoline, which then ignites by itself? What would the advantages and disadvantages be?
I would have thought that gasoline is too unstable under pressure, unless using an extremely high octane fuel. I’ll probably get shot down in flames now, but hey…that’s why I’m a nurse…mechanics isn’t really my forte!!
And from what I gather while regular gas may ignite (probably would actually) the diesel fuel has a much higher potential energy than regular gas and therefore you may get flame but it would either underpower the motor or not move the pistons at all (probably depends on the size of the piston vs the size of the crankshaft).
Glad you asked that, I always wondered about this myself and this was a good reason to look it up.
What I’ve always wondered about is why, if Diesels are so much more efficient than gasoline engines, do they have such dirty exhaust. Diesels blow clouds of soot, i.e., unburned carbon. I would think that this would lower the efficiency. What’s the deal here?
The problem is confusing cycle effciency with the type or category of the pollution emissions. Gasoline engines produce unburned hydrocarbons as well, but they are not the long-chain, soot-producing hydrocarbons that you see in diesel engines. It is the nature of the diesel fuel itself that trends towards producing these soot particles. Gasoline engine hydrocarbons are typically of smaller quantity, true, and said emissions are often invisible (unless there is very poor tuning or engine control). But overall the diesel cycle can still be more efficient.
The real disadvantage would be that the burning characteristics of the gasoline would lead to a lower combustion efficiency in such a system, unless somewhat radical changes were made in combustion chamber (and even cycle) design. I read once of a six-stroke engine that used such a principle in my grad class on IC engines, but can find no reference currently.
Most tanks without gas turbines are powered by diesels - and at least some of those diesels will run on quite a variety of combustibles, gasoline included.
Thanks… that was exactly what I was imagining. I guess it’s just trickier to get direct injection to work with gasoline than with diesel, so it hasn’t been done until recently.
was having a chat with my old man tonite, who’s a truckie. He drives a late model Scania 144. He’s got a mate who worked out, that on the basis of his fuel consumption versus the load he’s towing (approx 65 tonnes gross) that it would be the equivalent of a car getting around about 250 miles per gallon in so much as fuel consumption is concerned. So having said that…does this not make diesel engines more fuel efficient??
I owned a Toyota Diesel Pickup for a while (like about 300,000 miles) and once lent it to my son, who mistakenly put in about five gallone of gasoline. It didn’t seem to hurt it or cause any more (or less) smoke. (It didn’t smole all that bad in the first place.) BTW: it had zero guts; 0-60 in about half a day.
I have heard about vehicles that are intentionally multi-fuel (as opposed to my old truck), too. I think a lot of military vehicles are supposed to be that way these days. Now, as to what I don’t definitely know as a solid fact: I read on a site (which I think was either Federation of American Scientists or a link thereto) that in one of these ex-Soviet republics, someone’s military confiscated the output of someone else’s liquor plant to fuel their military vehicles. I also believe that Aviation Week once described the Su-25 “Frogfoot” attack aircraft as being multi-fuel capable so that it could operate from forward areas. Anthracite or one of the other brilliant chemists/physicists might know if it is possible to run a jet engine on diesel fuel, gasoline, or even liquor. And if it woud run, would it run safely enough to fly (produce about the same power ratings as before)? Wouldn’t it smoke like a pile of tires on fire? To quote the film “Dirty HArry”: “I gots to know!”
My 1st car was an 1980 300 Benz, and my current car is a 1984 300 Benz. >Dad has a thing for German cars< Anyway, both are diesels with approximately 21-gallon tanks. Going at an average rate of say 70mph, I can get about 30 miles to the gallon. I’d say that’s somewhat efficent. Both are in great shape too, although the 84 has a better exhileration rate then the 80, but that’s back to talking about engine designs and the such…
Well, I would seriously like to see what his mate actually worked out, because there is simply not that much of a difference between diesel and gasoline Otto-cycle efficiency. Not nearly that order of magnitude - we are talking about around or less than a 10% difference, IIRC from my rules of thumb from my Grad IC engines class.
Was he just working on a premise that it takes a fixed amount of horsepower to tow a 65 tonne load, and comparing that to what a car can tow, and trying to derive an efficiency from that? Because I cannot even begin to tell you why that analogy is wrong…
But on the premise that a car can tow a one tonne gross load, it would mean that 65 cars would be towing the load.
I think what I’m trying to say here, is that in-so-much as trucks are concerned, generally you would end up with less pollution from the one DIESEL vehicle towing a load as opposed to 65 GASOLINE cars towing the load.
You see, this is the wrong premise entirely. You are mixing the analogy of how many cars it would take to pull the load with whether or not diesel engines are more efficienct than gasoline engines. As per your previous post:
Herein, you are mixing here the real-world towing ability of a car versus a truck, with whether or not diesel engines are more fuel efficient. That is the error. If you said “Is it more efficient to use 65 cars to tow 1 tonne each, versus one truck to tow 65 tonnes”, then the answer is an obvious “yes”. But it has nothing to do with the relative merits and efficiency of the diesel engine versus the gasoline engine. It is also, by counter-example, much more efficient to have one gasoline truck tow 65 tonnes, than having 65 diesel cars each tow 1 tonne. See?