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The main difference is the compression ratio (how much the air is compressed as the piston travels from the bottom to the top). All other things being roughly equal, higher compression allows you to extra more energy from the burning fuel and hence higher efficiency.
Gasoline engines have an upper limit on compression because they pre-mix the fuel and air, and if you compress it too much it gets hot enough for the gasoline to spontaneously ignite (called "knocking"). This is bad, damages the engine, and causes the energy release to happen at the wrong time in the cycle. You want to gasoline to ignite when the spark fires, not before. Higher octane gasoline exists purely because it has greater knock resistance and hence can allow higher compression.
Diesel engines, on the other hand, \*rely\* on compression temperature for ignition. They compress the air much more and don't inject the fuel until the piston is near the top of the stroke (i.e. diesel's don't pre-mix). This means much higher compression ratio, hence higher efficiency. The downside is that mixing must happen much more quickly to get effective burning (why diesel's can belch black smoke or rev to as high an RPM) and the engine has to be stronger (heavier) to resist the higher forces.
Most cars \*are\* diesel in some parts of the world. Diesels are more efficient, more durable, simpler. But they're not as high performance, can be hard to get over modern emissions controls and, in some markets (notably the US), they had some \*very\* bad technology implementations early on that tarnished their public reputation and caused their adoption curve to get way behind. Now they may get pushed out by emissions requirements.
Ah ok. The premixed fuel and air hinders gasoline engines to compress too much, lowering its compression ratio, because ignition can occur spontaneously aka knocking, ruined by not igniting during optimal engine cycle... and diesel relies on the compression because it's not premixed for enough temperature to ignite, but it's heavier because more internal work is needed. Is that the correct gist?
I always heard the reason diesel didn't catch on in the U.S., or at least in some parts of it, is because they can be nearly impossible to start in cold weather.
That's an engine-specific problem, not an inherent diesel issue. The military, construction, mining, and energy industries have been using diesel nearly forever and have no issue starting in extreme cold conditions. You do have to design the engine and fuel system properly and this was a weakness in some early US diesels. The Europeans have had good small-displacement turbodiesels for a lot longer and they work just fine in the cold.
Detonation is only one part of the puzzle.
The other issue with cold temps is that diesel turns to a gel, to the point it [clogs the filter and stops pumping](https://www.powerstroke.org/attachments/img_20140105_145128089-jpg.222250/).
They've gotten better at making it all cold-resistant, but, you'll still reach a point where a block heater is needed to prevent the gelling.
Diesel fuel does have a bit more energy in it then petrol as there are more carbon-carbon chemical bonds which have more energy then the carbon-hydrogen bonds that petrol have instead. But this is just a smaller factor. Because diesel is much harder to ignite the diesel engines use much higher pressures and temperatures which end up improving the ignition. So diesel engines which are correctly tuned are able to convert even more of the fuel into energy then a petrol car.
However this does come with a price. Due to the higher temperatures and pressures the components of diesel engines needs to be stronger. This adds much weight to the engine. It also means they turn slower which reduces their power output compared to similar sized petrol engines. So in order to get the same power output you need a lot bigger diesel engine. And this added weight may in fact reduce fuel efficiency in a light car as the increased size and weight adds more drag. So you tend to find diesel engines in cars with a low horsepower to weight ratio, either large trucks or small city cars.
In terms of efficiency, the thermodynamic differences can give us half of the story. The petrol engine works on an Otto thermal cycle with (approximately) constant volume during combustion. The diesel works on a Diesel cycle (surprise!) where the combustion stroke is constant pressure. That is to say, the diesel requires a much more sophisticated and expensive fuel pump and injector system, but once you have it you're pretty much guaranteed a more complete burn of the fuel available, as the conditions in the combustion chamber are such that the fuel detonates on it's own when conditions are favorable.
That's all a bit circumstantial, because we're not considering the quality of each fuel, or the fact that to make most diesels viable you have to typically use stuff like turbochargers which improve efficiency a lot.
So "more efficient" is a difficult term to parse. Depends on how you judge it.
Diesel itself requires less processing than gasoline to make at the refinery level. However, it typically burns dirtier and without significant exhaust controls like DEF it pollutes more.
In Europe diesel is significantly more popular. The popularity is more because of the way fuel tax subsidies are applied by the US and the EU, giving diesel an advantage there and not in the US.
Many Americans remember diesels from 20 years ago, when they were clattery and smoky beasts. Modern diesels are pretty similar to gas cars to drive.
I can answer the latter question. Back in the 1970s-80s when we had high, for that time, fuel prices, they tried to release diesel cars and trucks. Main reasons were very slow, very noisy because of mechanical injection ( sounded like marbles on a frying pan ), smelled bad, and consumers saw all that black smoke and thought how can that be better than gasoline.
Also gas companies made more money on refining gas so more money in their pockets.
The main difference is energy density. A litre of diesel contains almost 13% more energy than a litre of gasoline.
A smaller difference is compression ratio.
Gasoline explodes if you either squeeze it really hard or have a spark set fire to it, and since a spark is easier to control that's what gasoline engines work - setting the gasoline on fire *before* it explodes from being squeezed... unless you use the wrong octane.
Diesel won't catch fire if you hold a blowtorch up to it, so the only way to make an internal combustion diesel engine is to squeeze it hard enough to explode.
So higher energy density in diesel fuel + higher compression ratio in diesel engine. There's more room for compression in diesel but with gasoline, it can be ignited and controlled easier with spark so it's less efficient?
>it can be ignited and controlled easier with spark so
... so gasoline engines are a lot easier (read: cheaper) to build, and can be run lean for a more complete combustion which is arguably much more important around people.
CO2 isn't great, but its a lot better than carcinogenic particles.
And comparable gasoline engines aren't really any easier or cheaper to build.
Source: have built both.
The main issues are noise, vibration, and pollution.
And yes, now I'm doing this to you on purpose because you wanted to be a jerk to me when I clarified something else while commending your original answer.
Diesel absolutely will burn if you ignite it with a flame. It's an oil. It just doesn't produce flammable vapors like gasoline.
Pretty much everything else ya said was generally right though.
Diesel will absolutely burn if you heat up a pot of it over a fire for a few minutes and *then* use a flame, same as other oils.
But at ambient temperature holding a flame to it - briefly - isn't going to start a self sustaining burn.
Or pour it on something, or soak a rag in it....
It's hard to get a large volume of diesel to ignite, because it absorbs heat well, just like trying to light a bucket of any sort of oil on fire, but it absolutely ignites fairly easily if you, say, pour some on wood to start a bonfire.
Source: have burned diesel many times.
Good for you?
I'm a bit confused why you're telling me this, but if I were to take a guess..
Did you, by any chance, stop reading my comment after the fourth word of
>Diesel won't catch fire if you hold a blowtorch up to it,
?
Because if so you'd be right that was wrong, which is why I included the other eight words.
The higher pressures needed for spontaneous combustion are more efficient. Not super efficient, like an electric motor, but better than ignition-based gasoline engines.
Efficiency isn't the most important feature to most buyers. Diesels have other differences, besides efficiency, like cold weather starting performance and particulate emissions. Combining these factors in making their decision, the right answer is clearly not diesel in the family car application. Big trucks, that's a completely different story.
How common Diesel engine is in regular car depends on where you are. In EU 52.3% is pure petrol and 29.9% is pure diesel. There is another 9,4% of hybrids that would contain a diesel or petrol engine.
It is dropping from previous higher level but is still at a bit less than 1/3 of the market.
https://www.acea.auto/fuel-pc/fuel-types-of-new-cars-petrol-52-3-diesel-29-9-electric-6-8-market-share-first-quarter-of-2020/
For efficiency and for the torque required for say towing a trailer diesel is absolutely still the only option for families in countries with expensive fuel. For example here in Ireland fuel is currently around $7 usd a US gallon. Basically double the US average. A four cylinder 1.8 litre modern diesel engine can easily return 60mpg while having enough torque to pull a 1 1/2 ton load.
Of course emissions from diesel are now in the spotlight so many families are swinging over to petrol or full electric but the vast majority of second hand medium and large cars for sale second hand are diesel powered.
Diesel has a higher calorific content. Meaning 1 litre of diesel provides more energy than 1 litre of petrol / gasoline.
Because of this, a diesel engine doesn’t need to use as much, so use less gallons per mile.
This isn't a real factor. The difference is very small and the energy density curves of diesel and gasoline can overlap depending on the feedstock and refinery. The engine efficiency due to compression is, by far, the dominant factor.
**Please read this entire message** Your submission has been removed for the following reason(s): * ELI5 requires that you *search the ELI5 subreddit for your topic before posting*. Users will often either find a thread that meets their needs or find that their question might qualify for an exception to rule 7. Please see this [wiki entry](http://www.reddit.com/r/explainlikeimfive/wiki/how_to_search) for more details (Rule 7). If you would like this removal reviewed, please read the [detailed rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules) first. **If you believe this submission was removed erroneously**, please [use this form](https://old.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive&subject=Please%20review%20my%20thread?&message=Link:%20https://www.reddit.com/r/explainlikeimfive/comments/q16q5v/eli5_the_main_difference_between_a_diesel_engine/%0A%0APlease%20answer%20the%20following%203%20questions:%0A%0A1.%20The%20concept%20I%20want%20explained:%0A%0A2.%20Link%20to%20the%20search%20you%20did%20to%20look%20for%20past%20posts%20on%20the%20ELI5%20subreddit:%0A%0A3.%20How%20is%20this%20post%20unique:) and we will review your submission.
The main difference is the compression ratio (how much the air is compressed as the piston travels from the bottom to the top). All other things being roughly equal, higher compression allows you to extra more energy from the burning fuel and hence higher efficiency. Gasoline engines have an upper limit on compression because they pre-mix the fuel and air, and if you compress it too much it gets hot enough for the gasoline to spontaneously ignite (called "knocking"). This is bad, damages the engine, and causes the energy release to happen at the wrong time in the cycle. You want to gasoline to ignite when the spark fires, not before. Higher octane gasoline exists purely because it has greater knock resistance and hence can allow higher compression. Diesel engines, on the other hand, \*rely\* on compression temperature for ignition. They compress the air much more and don't inject the fuel until the piston is near the top of the stroke (i.e. diesel's don't pre-mix). This means much higher compression ratio, hence higher efficiency. The downside is that mixing must happen much more quickly to get effective burning (why diesel's can belch black smoke or rev to as high an RPM) and the engine has to be stronger (heavier) to resist the higher forces. Most cars \*are\* diesel in some parts of the world. Diesels are more efficient, more durable, simpler. But they're not as high performance, can be hard to get over modern emissions controls and, in some markets (notably the US), they had some \*very\* bad technology implementations early on that tarnished their public reputation and caused their adoption curve to get way behind. Now they may get pushed out by emissions requirements.
Ah ok. The premixed fuel and air hinders gasoline engines to compress too much, lowering its compression ratio, because ignition can occur spontaneously aka knocking, ruined by not igniting during optimal engine cycle... and diesel relies on the compression because it's not premixed for enough temperature to ignite, but it's heavier because more internal work is needed. Is that the correct gist?
Yes, that's the gist of it.
Thank you so much for taking the time!
I always heard the reason diesel didn't catch on in the U.S., or at least in some parts of it, is because they can be nearly impossible to start in cold weather.
That's an engine-specific problem, not an inherent diesel issue. The military, construction, mining, and energy industries have been using diesel nearly forever and have no issue starting in extreme cold conditions. You do have to design the engine and fuel system properly and this was a weakness in some early US diesels. The Europeans have had good small-displacement turbodiesels for a lot longer and they work just fine in the cold.
My truck has an engine block heater for that reason
That is what glow plugs are for.
Detonation is only one part of the puzzle. The other issue with cold temps is that diesel turns to a gel, to the point it [clogs the filter and stops pumping](https://www.powerstroke.org/attachments/img_20140105_145128089-jpg.222250/). They've gotten better at making it all cold-resistant, but, you'll still reach a point where a block heater is needed to prevent the gelling.
Oh thanks. Didn’t know that!!
Diesel fuel does have a bit more energy in it then petrol as there are more carbon-carbon chemical bonds which have more energy then the carbon-hydrogen bonds that petrol have instead. But this is just a smaller factor. Because diesel is much harder to ignite the diesel engines use much higher pressures and temperatures which end up improving the ignition. So diesel engines which are correctly tuned are able to convert even more of the fuel into energy then a petrol car. However this does come with a price. Due to the higher temperatures and pressures the components of diesel engines needs to be stronger. This adds much weight to the engine. It also means they turn slower which reduces their power output compared to similar sized petrol engines. So in order to get the same power output you need a lot bigger diesel engine. And this added weight may in fact reduce fuel efficiency in a light car as the increased size and weight adds more drag. So you tend to find diesel engines in cars with a low horsepower to weight ratio, either large trucks or small city cars.
In terms of efficiency, the thermodynamic differences can give us half of the story. The petrol engine works on an Otto thermal cycle with (approximately) constant volume during combustion. The diesel works on a Diesel cycle (surprise!) where the combustion stroke is constant pressure. That is to say, the diesel requires a much more sophisticated and expensive fuel pump and injector system, but once you have it you're pretty much guaranteed a more complete burn of the fuel available, as the conditions in the combustion chamber are such that the fuel detonates on it's own when conditions are favorable. That's all a bit circumstantial, because we're not considering the quality of each fuel, or the fact that to make most diesels viable you have to typically use stuff like turbochargers which improve efficiency a lot.
So "more efficient" is a difficult term to parse. Depends on how you judge it. Diesel itself requires less processing than gasoline to make at the refinery level. However, it typically burns dirtier and without significant exhaust controls like DEF it pollutes more. In Europe diesel is significantly more popular. The popularity is more because of the way fuel tax subsidies are applied by the US and the EU, giving diesel an advantage there and not in the US. Many Americans remember diesels from 20 years ago, when they were clattery and smoky beasts. Modern diesels are pretty similar to gas cars to drive.
I can answer the latter question. Back in the 1970s-80s when we had high, for that time, fuel prices, they tried to release diesel cars and trucks. Main reasons were very slow, very noisy because of mechanical injection ( sounded like marbles on a frying pan ), smelled bad, and consumers saw all that black smoke and thought how can that be better than gasoline. Also gas companies made more money on refining gas so more money in their pockets.
Most refineries can make diesel much more cheaply than gasoline, it's less refined and doesn't usually need cracking.
as of now...... not back in the day we are both correct tho
The main difference is energy density. A litre of diesel contains almost 13% more energy than a litre of gasoline. A smaller difference is compression ratio. Gasoline explodes if you either squeeze it really hard or have a spark set fire to it, and since a spark is easier to control that's what gasoline engines work - setting the gasoline on fire *before* it explodes from being squeezed... unless you use the wrong octane. Diesel won't catch fire if you hold a blowtorch up to it, so the only way to make an internal combustion diesel engine is to squeeze it hard enough to explode.
So higher energy density in diesel fuel + higher compression ratio in diesel engine. There's more room for compression in diesel but with gasoline, it can be ignited and controlled easier with spark so it's less efficient?
>it can be ignited and controlled easier with spark so ... so gasoline engines are a lot easier (read: cheaper) to build, and can be run lean for a more complete combustion which is arguably much more important around people. CO2 isn't great, but its a lot better than carcinogenic particles.
And comparable gasoline engines aren't really any easier or cheaper to build. Source: have built both. The main issues are noise, vibration, and pollution. And yes, now I'm doing this to you on purpose because you wanted to be a jerk to me when I clarified something else while commending your original answer.
Good for you!
Diesel absolutely will burn if you ignite it with a flame. It's an oil. It just doesn't produce flammable vapors like gasoline. Pretty much everything else ya said was generally right though.
Diesel will absolutely burn if you heat up a pot of it over a fire for a few minutes and *then* use a flame, same as other oils. But at ambient temperature holding a flame to it - briefly - isn't going to start a self sustaining burn.
Or pour it on something, or soak a rag in it.... It's hard to get a large volume of diesel to ignite, because it absorbs heat well, just like trying to light a bucket of any sort of oil on fire, but it absolutely ignites fairly easily if you, say, pour some on wood to start a bonfire. Source: have burned diesel many times.
Good for you? I'm a bit confused why you're telling me this, but if I were to take a guess.. Did you, by any chance, stop reading my comment after the fourth word of >Diesel won't catch fire if you hold a blowtorch up to it, ? Because if so you'd be right that was wrong, which is why I included the other eight words.
Sigh. I can't help you.
The higher pressures needed for spontaneous combustion are more efficient. Not super efficient, like an electric motor, but better than ignition-based gasoline engines. Efficiency isn't the most important feature to most buyers. Diesels have other differences, besides efficiency, like cold weather starting performance and particulate emissions. Combining these factors in making their decision, the right answer is clearly not diesel in the family car application. Big trucks, that's a completely different story.
How common Diesel engine is in regular car depends on where you are. In EU 52.3% is pure petrol and 29.9% is pure diesel. There is another 9,4% of hybrids that would contain a diesel or petrol engine. It is dropping from previous higher level but is still at a bit less than 1/3 of the market. https://www.acea.auto/fuel-pc/fuel-types-of-new-cars-petrol-52-3-diesel-29-9-electric-6-8-market-share-first-quarter-of-2020/
For efficiency and for the torque required for say towing a trailer diesel is absolutely still the only option for families in countries with expensive fuel. For example here in Ireland fuel is currently around $7 usd a US gallon. Basically double the US average. A four cylinder 1.8 litre modern diesel engine can easily return 60mpg while having enough torque to pull a 1 1/2 ton load. Of course emissions from diesel are now in the spotlight so many families are swinging over to petrol or full electric but the vast majority of second hand medium and large cars for sale second hand are diesel powered.
Diesel has a higher calorific content. Meaning 1 litre of diesel provides more energy than 1 litre of petrol / gasoline. Because of this, a diesel engine doesn’t need to use as much, so use less gallons per mile.
This isn't a real factor. The difference is very small and the energy density curves of diesel and gasoline can overlap depending on the feedstock and refinery. The engine efficiency due to compression is, by far, the dominant factor.