Would a diesel-electric hybrid be better mpg than gas-electric hybrid

Main Question or Discussion Point

Given the earlier thread, why not combine diesel with electric motors?

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Ivan Seeking
Staff Emeritus
Gold Member
We can kill three birds with one stone.

Diesel hybrid concept car also taps the sun
Ford says all-wheel-drive Reflex can get 65 mpg
http://www.msnbc.msn.com/id/10788292/

Note that trains have been running diesel over electric for decades.

I would almost assure that. Though from everything ive read, a detached diesel motor/generator would be more efficent than a parallel drive design, as a detached generator would let the diesel run at peak efficiency constantly rather than constantly changing rpm and efficiency

I would almost assure that. Though from everything ive read, a detached diesel motor/generator would be more efficent than a parallel drive design, as a detached generator would let the diesel run at peak efficiency constantly rather than constantly changing rpm and efficiency
This is called a series hybrid and is the type of hybrid that most hybrid auto manufacturers are now focusing on.

While a diesel based hybrid may get better fuel economy than a gasoline engine, it doesn't necessarily make it better. Diesel engines generate a lot of pollution, which is why they are not very common in the US for commercial vehicles. Gasoline is a much more cleaner burning fuel which is why its most commonly used. Future series hybrid electric/gas cars will probably use stratified ignition engines, or something close to it, to try and take advantage of the benefits the diesel cycle offers, but still using the clean burning fuel, gasoline.

Ivan Seeking
Staff Emeritus
Gold Member
This is called a series hybrid and is the type of hybrid that most hybrid auto manufacturers are now focusing on.

While a diesel based hybrid may get better fuel economy than a gasoline engine, it doesn't necessarily make it better. Diesel engines generate a lot of pollution, which is why they are not very common in the US for commercial vehicles.
Ultra-low-sulfur fuels now required in the US contain 15 ppm sulfur, compared to ~500 ppm in the older fuels. Particulate traps used in late-model diesels further reduce emissions. Diesels are very common in commercial, but not private vehicles, primarily because diesels could not meet emission standards. They do now.

Assuming Exxon et al manage to produce biodiesel from algae [$600 million recently invested by Exxon alone], diesel CO2 emissions would be reduced to near zero. Also, biodiesel contains no sulfur. Last edited: brewnog Science Advisor Gold Member While a diesel based hybrid may get better fuel economy than a gasoline engine, it doesn't necessarily make it better. Diesel engines generate a lot of pollution, which is why they are not very common in the US for commercial vehicles. Gasoline is a much more cleaner burning fuel which is why its most commonly used. Future series hybrid electric/gas cars will probably use stratified ignition engines, or something close to it, to try and take advantage of the benefits the diesel cycle offers, but still using the clean burning fuel, gasoline. Sorry, this is cobblers. Do you have anything to back up the claim that Diesel engines "generate a lot of pollution"? Have you actually even compared a modern gasoline engine with its Diesel counterpart, in terms of brake specific NOx/CO/HC/PM/CO2 emissions, or do you just think "old lorries make smoke, so all Diesel engines must be terrible"? Sorry, this is cobblers. Do you have anything to back up the claim that Diesel engines "generate a lot of pollution"? Have you actually even compared a modern gasoline engine with its Diesel counterpart, in terms of brake specific NOx/CO/HC/PM/CO2 emissions, or do you just think "old lorries make smoke, so all Diesel engines must be terrible"? No it isn't. Yes I do. Yes, and no I don't. A simple google search reveals lots of info about how gasoline is cleaner than diesel.... Most of today’s hybrids and even some conventional gasoline vehicles are considerably cleaner... http://www.hybridcenter.org/hybrid-watchdog-hybrid-vs-diesel.html ...it takes about 25% more oil to make a gallon of diesel fuel than a gallon of gasoline, so we should really look at how a vehicle does on fuel efficiency in terms of "oil equivalents." ...when it comes to smog-forming pollutants and toxic particulate matter, also known as soot, today's diesels are still a lot dirtier than the average gasoline car. US tailpipe standards for diesel cars, which have historically been weaker than those for gasoline cars, are being updated to force diesel and gasoline vehicles to meet the same set of emissions standards. The tiered structure of the new "Tier 2" standards, however, allows automakers to produce some cars that release two times more soot and smog-forming pollution than the average new vehicle and still meet their targets. Also, until the standards are fully implemented in 2009, existing loopholes allow some cars to pollute even more. Making a gallon of diesel fuel requires 25% more oil and emits 17% more heat-trapping greenhouse gases than gasoline reformulated with MTBE. Similarly, diesel requires 17% more oil and emits 18% more heat-trapping gases than gasoline reformulated with ethanol. This means that diesel fuel's advantages from its higher per-gallon energy content and better performance on greenhouse gases are partially offset by the impact of diesel's fuel-production process. http://www.grinningplanet.com/2005/04-12/diesel-vs-gasoline-article.htm We know from before that passenger cars with diesel engines emit, depending on their condition, about 20 percent less of the greenhouse gas carbon dioxide (CO2) per kilometer than the same car type with gasoline engines. However, besides CO2, the exhaust contains a number of other components such as nitrogen oxide (NOx), carbon monoxide (CO), and particles. With the exception of CO, emissions of these components are generally higher from diesel cars, and lead to lower air quality and negative health effects. http://www.cicero.uio.no/fulltext/index_e.aspx?id=6871 The text book right in front of me, (An Introduction to Combustion, by: S.R. Turns) also supports my claim. While the sulfur content in diesel has decreased, it hasn't changed the fundamental ways diesel engines operate. Unless flame temperatures can be lowered, fuel can be better atomized, or diesel production be made more efficient, I don't see how diesel could ever be cleaner than gasoline. If I'm missing something or don't know of some technological breakthrough that has made this possible please enlighten me. Last edited by a moderator: Assuming Exxon et al manage to produce biodiesel from algae [$600 million recently invested by Exxon alone], diesel CO2 emissions would be reduced to near zero. Also, biodiesel contains no sulfur.
Biodiesel is still far from reality. Even so, diesel engines whether biodiesel or not, still generate pollutants. Yes there is less CO2, but theres also plenty of CO, NOx, HC, etc. But if biodiesel creates less of the pollutants than gasoline, I don't know, I've heard mixed information. Do you have any info on this?

EDIT: Some good info about biodiesel,
but diesel still has a long way to go to match the emissions performance of the many fuel-efficient conventional and hybrid electric gasoline-powered vehicles on the road today.
http://www.ucsusa.org/clean_vehicles/technologies_and_fuels/biofuels/biodiesel-basics.html#Biodiesel_versus_gasoline_in_cars_and_li [Broken]

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Driving a modern, small displacement diesel can be fun. I assume they'd make a fine hybrid animal given their advantages in torque and economy.

Of course, pollution is in they eye of the beholder. Europe loves diesel cars and things there are clean, especially Germany.

Yes, diesels pollute an order of magnitude more than gasoline. But if you consider the 'problem' of automotive gasoline pollution equivalent to the 'problem' of time dilation at highway speeds (as I do), you should have no moral hangups with a diesel.

brewnog
Gold Member
Topher925, are you really standing by your link which holds the Tier 2 on-road regulation as being consistent with a modern on-road (Tier 4b, Stage V etc) Diesel engine?

Given the earlier thread, why not combine diesel with electric motors?
As already pointed out, trains have been utilizing diesel/electric power for a very long time.

They also have jet turbine/electric systems. Which may not be as practical in a family sedan

Ivan Seeking
Staff Emeritus
Gold Member
Biodiesel is still far from reality.
Not true. Biodiesel is in common use now. What is not sustainable is the source - that typically being soy and canola, cotton, and a few other crops. That is why algae-derived fuels are the key. Algae can produce five to ten times the yield per acre-year, or more, compared to any other fuel crop.

Even so, diesel engines whether biodiesel or not, still generate pollutants. Yes there is less CO2
Not less CO2; I said virtually no CO2. In principle, biodiesel emits no new CO2 - only that absorded from the atmosphere by the fuel source [crop]. It is carbon neutral. No other fuel option can make the same claim, less perhaps ethanol, which has other serious drawbacks and is completely unsustainable. The best option for a sustainable source of ethanol may be...algae.

.. but theres also plenty of CO, NOx, HC, etc. But if biodiesel creates less of the pollutants than gasoline, I don't know, I've heard mixed information. Do you have any info on this?

EDIT: Some good info about biodiesel,

http://www.ucsusa.org/clean_vehicles/technologies_and_fuels/biofuels/biodiesel-basics.html#Biodiesel_versus_gasoline_in_cars_and_li [Broken]

Note that only the limit of your graph shows the results for pure biodiesel.

Biodiesel is notoriously low in emissions, less NOX emission, which are a bit higher than petrodiesel. Beyond that, the emissions from any particular biodiesel fuel depends on the source of the fuel. Oils low in saturated fats, like oils from algae, produce the cleanest fuels. Being high in saturated fats, soy produces a relatively unclean biodiesel, which is what most studies would have used.

There are two factors to consider regarding the energy content of petrodiesel. First is the amount of energy contained in a gallon of fuel. While this tracks approximately 1:1 with the amount of crude required, we transport ~25% less fuel to the market for a given amount of energy. Beyond the reduced distribution energy costs, diesel engines are more efficient than IC engines.

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mheslep
Gold Member
No it isn't. Yes I do. Yes, and no I don't. A simple google search reveals lots of info about how gasoline is cleaner than diesel....
[...]
If I'm missing something or don't know of some technological breakthrough that has made this possible please enlighten me.
Topher, after citing http://www.cicero.uio.no/fulltext/index_e.aspx?id=6871":

Cicero article said:
[...]We know from before that passenger cars with diesel engines emit, depending on their condition, about 20 percent less of the greenhouse gas carbon dioxide (CO2) per kilometer than the same car type with gasoline engines. However, besides CO2, the exhaust contains a number of other components such as nitrogen oxide (NOx), carbon monoxide (CO), and particles. With the exception of CO, emissions of these components are generally higher from diesel cars, and lead to lower air quality and negative health effects.
one can see your first blanket answer to the OP's query about gasoline versus diesel pollutants in this engineering forum was incorrect on a per km distance for CO2 and CO. It is known that Bio-diesel burns cleaner than petrol diesel. Brownog also drew your intention to the new Tier 2 bin 5 EPA standards, which reduce particulates to nearly nothing.

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Mech_Engineer
Gold Member
This thread really got way off on a tangent...

In response to the OP's question- yes, diesel hybrids (parallel or series) will tend to get better mileage than a gasoline one. This is due to the fact that turbo-diesel engines are generally more efficient than a gasoline engine because of their high compression ratio and turbocharging. Additionally, Diesel has approximately 10% more stored energy per volume than gasoline, adding to a diesel vehicle's MPG rating (but not overall energy efficiency).

The reason they aren't widely available is mainly cost. A turbodiesel engine is more expensive than a naturally aspirated gasoline one, which can make a big difference when you're putting a lot of expensive hybrid equipment on a vehicle as well.

Topher925, are you really standing by your link which holds...
No. I'm not familiar with the politics, but only the science of how thermal engines work.

Not true. Biodiesel is in common use now.
I did not know this. Where is it being used?

Biodiesel is notoriously low in emissions, less NOx emission, which are a bit higher than petrodiesel.
Yes, but keep in mind that NOx emissions are one of the most harmful emissions created. The idea behind a closed CO2 cycle is appealing but you can't ignore the other harmful products that are generated as well.

Topher, after citing http://www.cicero.uio.no/fulltext/index_e.aspx?id=6871":

one can see your first blanket answer to the OP's query about gasoline versus diesel pollutants in this engineering forum was incorrect on a per km distance for CO2 and CO. It is known that Bio-diesel burns cleaner than petrol diesel. Brownog also drew your intention to the new Tier 2 bin 5 EPA standards, which reduce particulates to nearly nothing.
I'm sorry, I don't understand your logic??? I never stated that diesels produced more CO2 or CO, I merely stated that diesels produce more pollution than their gasoline counterparts, which according to my sources they do. I have seen no information that suggests otherwise.

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mheslep
Gold Member
I'm sorry, I don't understand your logic??? I never stated that diesels produced more CO2 or CO,
Meaning what, that neither CO2 nor CO are pollution? In post 3 you made the blanket statement
post 3 said:
Diesel engines generate a lot of pollution, which is why they are not very common in the US for commercial vehicles. Gasoline is a much more cleaner burning fuel which is why its most commonly used.
[my highlights]Without attaching a qualification that assertion is incorrect. The truth is the pollution profile, diesel vs gasoline, is mixed for unit of energy delivered. Also, this considers only the gasses at the point where they are exhausted directly from the cylinder. Post processing in the most recent engines might change the mix decidedly in favor of diesel, I dunno.

I merely stated that diesels produce more pollution than their gasoline counterparts, which according to my sources they do. I have seen no information that suggests otherwise.
Look, I could point to those same sources and naively make the reverse statement because I happened to focus only on CO2 and CO in favor of diesel.

brewnog
Gold Member
No. I'm not familiar with the politics, but only the science of how thermal engines work.
My post wasn't about the politics, it was about the technology which is now being used to ensure that NOx and PM emissions of Diesel vehicles are now a tiny fraction of what they were a few years ago; and in some cases can be cleaner than the air being drawn into the engine. Saying Diesel engines are "more polluting than gasoline" is just misguided if you're comparing a smokey old truck with a Euro V (or equivalent) petrol. I could just as easily claim that Diesel is "cleaner" because the CO emission limits now are half for Diesel what they are for gasoline. The thing is, if you only look at selected pollutants, it's easy to claim that either is somehow inherently "less polluting" than the other. Look at the bigger picture.

Ranger Mike
Gold Member
well said Brewnog....I share your views on the diesel

btw ..you are invited to my Earth Day celebration next month..( bring your old engine oil and tires for our bon fire)

mheslep
Gold Member
I could just as easily claim that Diesel is "cleaner" because the CO emission limits now are half for Diesel what they are for gasoline. ...
Well said, as are the CO2 emissions, the benzene emissions, etc (lower, not necessarily half)

mgb_phys
Homework Helper
Modern diesel engines are pretty much as clean as gasoline.
They produce less CO2 and you get more miles/gallon, PM are about the same as gasoline although there is a concern that the mix of particle sizes produced by diesels is more harmful.

A plus of diesel for hybrids is that you can make very efficent small diesels - the original european version of the smart car had a 800cc diesel. Diesels are also simpler and you can turn them off and restart them with less loss of efficency than a gasoline engine.

The big problem is acceptance in the US, it's hard to get fuel because nobody has diesels and nobody buys diesels because you can't get fuel. Also opinions take a long time to change.

Another factor is the way pollution is calculated, in the Eu it's generally g/km so the easiest way to reach the level is to increase the number of km/litre by building smaller engines. In the US it was always pollutant concentration so the easiest way to meet the target was to build bigger engines. This is changing though.

The below is a mix of fact and my not so humble opinion.

Modern TurboDiesel engines > Modern Petrol engines

In terms of fuel economy, power where it matters, overall emissions PER KM TRAVELLED (g/km). It's the fact that you can turbocharge them with pretty much no drawbacks allowing easier downsizing that give them the edge. Also common rail injection too.

And it's a sad sad sad day for me to admit that. I'm still not going to be buying a diesel, as they are just not cool.

mheslep
Gold Member
Modern diesel engines are pretty much as clean as gasoline.
I had hoped that the preceding posts would have prompted more careful sourced based statements, and less sweeping claims with no sources. Anyway:

They produce less CO2 and you get more miles/gallon, PM are about the same as gasoline although there is a concern that the mix of particle sizes produced by diesels is more harmful.
Clearly, the new environmental standards (EPA and EU) require greatly reduced PM emissions over a decade ago. That's still a long way from asserting diesel PM is "about the same as gasoline." Can you verify that?
A plus of diesel for hybrids is that you can make very efficient small diesels - the original european version of the smart car had a 800cc diesel. Diesels are also simpler and you can turn them off and restart them with less loss of efficiency than a gasoline engine.
Smaller but not lighter weight that I can see. That high compression comes with a weight cost, and thus a $cost. Observe that we do not see diesel back pack leaf blowers, or diesel push lawn mowers. mheslep Gold Member The below is a mix of fact and my not so humble opinion. Modern TurboDiesel engines > Modern Petrol engines How? More letters in the name? mgb_phys Science Advisor Homework Helper That's still a long way from asserting diesel PM is "about the same as gasoline." Can you verify that? http://www.euractiv.com/en/transport/euro-5-emissions-standards-cars/article-133325 The 2009 euro5 regs requires diesels to have the same PM emissions as gasoline Merc and VW both have diesels in the US that already beat the Euro5 specs. Smaller but not lighter weight that I can see. That high compression comes with a weight cost, and thus a$ cost. Observe that we do not see diesel back pack leaf blowers, or diesel push lawn mowers.
But negligible weight difference for a car, you don't see 4stroke leaf blowers with cats either.

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mheslep
Gold Member
http://www.euractiv.com/en/transport/euro-5-emissions-standards-cars/article-133325
The 2009 euro5 regs requires diesels to have the same PM emissions as gasoline
Merc and VW both have diesels in the US that already beat the Euro5 specs.
Yes I was aware that the regulations for each had come roughly into line from an earlier EPA reference; that doesn't necessarily mean diesel matches gasoline PM emissions; gasoline may much less than the regulation.