# Diesel autos are a illusion of increased efficiency **URL:** \ **Category:** Great Debates\ **Created:** [June 5, 2011, 8:30pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429 "2011-06-05T20:30:11Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Martin\_Hyde](https://avatars.discourse-cdn.com/v4/letter/m/47e85d/32.png) [@Martin\_Hyde](https://boards.straightdope.com/u/Martin_Hyde)\ **Post date:** [June 6, 2011, 1:16am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/21 "2011-06-06T01:16:10Z")
> [@DSeid](#): > > The problem is it depends on the variable. NOx? CO2? Particulates? Total cost of ownership over the usual five years? All I will say is that as an individual car buyer the major reason I’ve not seriously considered any of the new diesel offerings is the increased price would never be offset for me through the decreased annual fuel cost. ---
**Author:** ![Martin\_Hyde](https://avatars.discourse-cdn.com/v4/letter/m/47e85d/32.png) [@Martin\_Hyde](https://boards.straightdope.com/u/Martin_Hyde)\ **Post date:** [June 6, 2011, 1:19am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/22 "2011-06-06T01:19:14Z")
> [@kanicbird](#): > > It’s a bit deceptive. You may be able to make a car that runs on liquid hydrogen that gets 170 mpg while the equivalent gas car gets 30. It would not be a fair way to compare efficiency to do it mpg. This is a exaggeration to show a point that if you compare it by mpg you are going to come up with a unfair advantage to the fuel with the higher energy content. Diesel is fairly close enough that most people don’t look, though it gives a non negligible artificially high mpg to diesels. My question is how much? > > Electric cars are given a mpg equivalent of gasoline, what i am stating is that diesel needs this also. So what you are saying is that because diesel engines are intrinsically more efficient it is deceptive to say diesel fuel is more efficient? I guess I’m not really sure what point you are trying to make. People actually have mpg figures for diesels because you _actually fill diesels up just like gasoline cars_. It’s not like a plug-in electric, since consumers know that 40 mpg means they will need to refill every ~450 miles it means they can use that mpg to guesstimate what sort of monthly fuel bill they are looking at. I think that is the reason it is used and it is entirely fair to compare it to gasoline because what a consumer is comparing is their monthly fuel expenditure at the gas pump and not anything else, and for that purpose the mpg do compare accurately and in the manner expected. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [June 6, 2011, 2:16am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/23 "2011-06-06T02:16:54Z")
> [@Martin\_Hyde](#): > > So what you are saying is that because diesel engines are intrinsically more efficient it is deceptive to say diesel fuel is more efficient? I guess I’m not really sure what point you are trying to make. Because diesel fuel contains more energy and cost more is it deceptive to compare mpg numbers directly with gasoline cars. > [@](#): > > People actually have mpg figures for diesels because you _actually fill diesels up just like gasoline cars_. It’s not like a plug-in electric, since consumers know that 40 mpg means they will need to refill every ~450 miles it means they can use that mpg to guesstimate what sort of monthly fuel bill they are looking at. I think that is the reason it is used and it is entirely fair to compare it to gasoline because what a consumer is comparing is their monthly fuel expenditure at the gas pump and not anything else, and for that purpose the mpg do compare accurately and in the manner expected. It is the comparison am trying to equate. How do you compare: diesel car 42 mpg diesel vs. gas car 34 mpg gasoline It would be easier to compare: diesel car 37 mpg gasoline equivalent vs. gasoline car 34 mpg gasoline ---
**Author:** ![Martin\_Hyde](https://avatars.discourse-cdn.com/v4/letter/m/47e85d/32.png) [@Martin\_Hyde](https://boards.straightdope.com/u/Martin_Hyde)\ **Post date:** [June 6, 2011, 2:20am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/24 "2011-06-06T02:20:54Z")
> [@kanicbird](#): > > It would be easier to compare: > diesel car 37 mpg gasoline equivalent vs. gasoline car 34 mpg gasoline Huh, but then people wouldn’t know how many miles they could drive on a tank of diesel… ---
**Author:** ![Raygun99](https://avatars.discourse-cdn.com/v4/letter/r/6f9a4e/32.png) [@Raygun99](https://boards.straightdope.com/u/Raygun99)\ **Post date:** [June 6, 2011, 2:31am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/25 "2011-06-06T02:31:43Z")
> [@kanicbird](#): > > Because diesel fuel contains more energy and cost more is it deceptive to compare mpg numbers directly with gasoline cars. This is precisely the point as to why diesel is more efficient. What you are saying is like complaining about the fact a steak contains more energy than celery, and if you adjusted for that fact, celery contains as much energy as steak. > [@](#): > > It is the comparison am trying to equate. How do you compare: > diesel car 42 mpg diesel vs. gas car 34 mpg gasoline Well, the car running on diesel can travel 42 miles on one gallon of its fuel, while the the car running on gasoline can travel 34 miles. ---
**Author:** ![Ravenman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ravenman/32/2929_2.png) [@Ravenman](https://boards.straightdope.com/u/Ravenman)\ **Post date:** [June 6, 2011, 2:40am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/26 "2011-06-06T02:40:28Z")
> [@kanicbird](#): > > It would be easier to compare: > diesel car 37 mpg gasoline equivalent vs. gasoline car 34 mpg gasoline Do we get to mark down hybrids because they cheat and use an electric engine that is much more efficient than an internal combustion one? Why should a hybrid get credit for using its electric drive when it is clear that it is like 3 times more efficient than a gasoline engine? That’s not fair. It should be “hybrid car 30 miles gasoline engine only vs. gas car 34 mpg gasoline.” Then we will see through the corporate claims that hybrids are more efficient. By these measures, I’m dying to see the **kanicbird** Olympics. In the weightlifting competitions, are the weights adjusted so that stronger people and weaker people are more equal? Like, Sven the Swedish Stallion can lift 330 kg, but his weights are marked down by an arbitrary percentage so that Paul the Puny Pole wins the competition by lifting 240 kg. Can’t have Sven be more full of energy than Paul, of course. Then it isn’t an apples to apples comparison. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [June 6, 2011, 3:01am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/27 "2011-06-06T03:01:05Z")
> [@Ravenman](#): > > Do we get to mark down hybrids because they cheat and use an electric engine that is much more efficient than an internal > combustion one? How do hybrids cheat? they list a mpg gas equivalent so you can compare hybrids and pure electrics directly to gasoline cars. You can see the gas model is 34 mpg, the hybrid is 49 mpg gas equivalent. Diesels ‘cheat’ because they don’t give you that equivalent, and that equivalent number would be lower then the mpg stated for the diesel vehicle. ---
**Author:** ![Ravenman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ravenman/32/2929_2.png) [@Ravenman](https://boards.straightdope.com/u/Ravenman)\ **Post date:** [June 6, 2011, 3:10am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/28 "2011-06-06T03:10:16Z")
> [@kanicbird](#): > > How do hybrids cheat? they list a mpg gas equivalent so you can compare hybrids and pure electrics directly to gasoline cars. You can see the gas model is 34 mpg, the hybrid is 49 mpg gas equivalent. By using regenerative braking, hybrids use more than just gasoline to generate power for the motor. If you’re going to insist the the fuel efficiency of diesels be modified because diesel has more energy than gasoline, then it is only fair that hybrids be evaluated without regenerative braking, which gives it an unfair advantage over gas powered vehicles. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [June 6, 2011, 3:14am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/29 "2011-06-06T03:14:08Z")
> [@Ravenman](#): > > By using regenerative braking, hybrids use more than just gasoline to generate power for the motor. > > If you’re going to insist the the fuel efficiency of diesels be modified because diesel has more energy than gasoline, then it is only fair that hybrids be evaluated without regenerative braking, which gives it an unfair advantage over gas powered vehicles. Hybrids use gasoline, diesels don’t. the technology or efficiency is not the issue, it’s the use of energy. By consuming a gallon of diesel you have used more energy then if you consumed a gallon of gas. That energy comes from the ground and there is only so much pumped out. You could also state it is terms of $/mile, which like energy used/mile diesels would be marked lower then their mpg suggests. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [June 6, 2011, 3:22am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/30 "2011-06-06T03:22:26Z")
To put it another way, lets say you could somehow compress liquid gasoline into a smaller volume. You have one car that uses standard gasoline another that uses the compressed gasoline. You marketed the compressed gasoline as having near hybrid mpg’s. but it’s high mpg’s is only because it uses the compressed fuel. In reality the car using the compressed gasoline uses the same energy per mile as the other. Now in the case of diesel yes there is apparently some efficiency gains, but part of the increase is because diesel is a more compact form of gasoline (in terms of energy storage). Using a more compact fuel is not increasing combustion efficiency. ---
**Author:** ![Ravenman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ravenman/32/2929_2.png) [@Ravenman](https://boards.straightdope.com/u/Ravenman)\ **Post date:** [June 6, 2011, 3:22am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/31 "2011-06-06T03:22:56Z")
Regenerative braking essentially turns brake heat into electricity (yes, I’m being a bit broad with the terms). Diesels don’t turn friction into energy. Normal cars don’t turn friction into energy. Why should diesels get credit for a power source that isn’t gasoline if diesels can’t do the same? Answer: MPG is simply a measure of how far a volume of a fuel gets you. If you want to go off and invent a BTUs per furlong efficiency measurement, be my guest - but it will be irrelevant to everyone. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [June 6, 2011, 3:23am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/32 "2011-06-06T03:23:42Z")
After reading this thread, all I can say is my brain hurts. ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [June 6, 2011, 3:44am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/33 "2011-06-06T03:44:49Z")
> [@DSeid](#): > > The problem is it depends on the variable. NOx? CO2? Particulates? Total cost of ownership over the usual five years? you left out hp and torque. A diesel will haul more weight which translates into more people/cargo. A minivan with a turbo-diesel will have the fuel efficiency when needed and hp when called upon. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [June 6, 2011, 3:54am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/34 "2011-06-06T03:54:15Z")
> [@Raygun99](#): > > This is precisely the point as to why diesel is more efficient… Not quite. Diesels are more efficient both because diesel fuel contains more energy (and more carbon) per unit volume _and_ because the engine burns that fuel more efficiently. I started a [thread similar to this one](http://boards.straightdope.com/sdmb/showthread.php?t=606023&highlight=diesel) before, and as far as CO2 goes, we found some interesting numbers in [this tech sheet](http://www2.fordconnection.com/fordconnection/multimedia/gbr_en/00w/yyh/gbr_en00wyyhfuelspc.htm). Short version is that per distance traveled the EU diesels produce 80 to 87% of CO2 emissions than do similar petrol models. Noted is that these vehicles do not meet US particulate standards. And that a _significantly_ larger fraction of oil going into diesel production would force inefficiencies into the system and [increase GHGs](http://ie.jrc.ec.europa.eu/publications/scientific_publications/2004/EUR%2021378%20EN.pdf). Reporting CO2 emissions per mile as well as mpg (or perhaps better gallon per mile) similar to how that tech page does would be a good approach for the US to emulate. So figure 15% less CO2 produced per mile well to wheel right now. Given the significantly increased expense of diesel vehicles (and of diesel fuel), and that diesels are a fairly mature technology that have had their days of tax credit supports, it seems that there are going to be and are already more cost effective ways to achieve that. Albeit, as **Magiver** points out, they also have better utility for some functions than other vehicles do. For people who tow and who especially who tow long distances, diesels really can’t be beat, for example. ---
**Author:** ![The\_Flying\_Dutchman](https://avatars.discourse-cdn.com/v4/letter/t/9fc348/32.png) [@The\_Flying\_Dutchman](https://boards.straightdope.com/u/The_Flying_Dutchman)\ **Post date:** [June 6, 2011, 4:13am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/35 "2011-06-06T04:13:52Z")
> [@kanicbird](#): > > Can I say now that diesel fueled autos that toute environmental benefits and high MPG’s is nothing but a lie told by marketing departments to the potential buyers of said vehicles? I can’t address the argument regarding environmental considerations, but I can say I don’t care what’s behind the miles per gallon stats for diesel and gas. All I know is if I can go farther on a gallon of diesel than I can on a gallon of gas than I’m better off with diesel. I’ve owned 2 Ford 250s in my life, gas and diesel, and I can tell you that diesel goes a lot further. And if you are into marine craft, the financial benefit of diesel over gas wrt to consumption is even way more dramatic. I never listen to marketing departments. I suspect that the major auto companies rather sell gas anyway given that they don’t manufacture their own diesel engines. (I’m assuming that hasn’t changed from 20 years ago) ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [June 6, 2011, 4:31am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/36 "2011-06-06T04:31:04Z")
> [@kanicbird](#): > > That energy comes from the ground and there is only so much pumped out. AFAIK, all gasoline is ‘pumped from the ground’. Not all Diesel is. ---
**Author:** ![The\_Second\_Stone](https://avatars.discourse-cdn.com/v4/letter/t/bbe5ce/32.png) [@The\_Second\_Stone](https://boards.straightdope.com/u/The_Second_Stone)\ **Post date:** [June 6, 2011, 4:33am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/37 "2011-06-06T04:33:22Z")
A given measure of diesel fuel has more energy in it than the same measure of gasoline. This is a fact. Quite a bit more energy. So identical vehicles with similar size engines, like the aforementioned Jetta’s with 2.0 liters, will see the diesel getting much higher miles per gallon. That is also a fact. The only efficiency here is that the good Lord more efficiently packed chemical reaction energy into diesel than into gasoline. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [June 6, 2011, 4:43am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/38 "2011-06-06T04:43:10Z")
> [@The\_Second\_Stone](#): > > The only efficiency here is that the good Lord more efficiently packed chemical reaction energy into diesel than into gasoline. I claim copyright on the following phrase: GOD LIKES DIESEL ---
**Author:** ![The\_Second\_Stone](https://avatars.discourse-cdn.com/v4/letter/t/bbe5ce/32.png) [@The\_Second\_Stone](https://boards.straightdope.com/u/The_Second_Stone)\ **Post date:** [June 6, 2011, 4:44am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/39 "2011-06-06T04:44:40Z")
Well, I get Vin Diesel is God like. FWIW. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [June 6, 2011, 5:06am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/40 "2011-06-06T05:06:54Z")
kudos to **matt’s** post #10. Saves me all the typing I usually have to do describing how diesels work. To the OP: The limit on spark ignition compression ratio is due to detonation, not NOx emissions. SI compression ratios have been creeping upward as NOx regulations have tightened. NOx is fairly easy handle in a catalytic converter, as it has the oxygen and energy needed for reduction, and that is why gasoline engine makers are able to run the compression ratios up to the detonation threshold. More than a few modern Diesels are also running catalytic converters to reduce NOx. Finally one of the ways of reducing NOx is exhaust gas recirculation (EGR) which actually improves mid throttle economy, as it allows decreased manifold vacuum, which lowers pumping losses. If gasoline engines were more efficient, trains, over-the-road trucking, and ships would be burning gasoline. [Previous page](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429.md?page=1) [Next page](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429.md?page=3)