February 29, 2012

What's so clean about diesel?

Say goodbye to the image of Mack trucks belching a large black cloud of exhaust wherever they go. With the arrival of ultra-low-sulphur ("clean") diesel at US pumps, and the development of power-boosting and exhaust-scrubbing technologies, diesel-powered passenger cars are cleaner than their gasoline counterparts on just about all emissions - and run at a higher fuel efficiency.

In the realm of the internal combustion engine, it's hard to beat a diesel engine for its simplicity: there is a piston (one of four, or six) that compresses air to 1/25th or so of its original volume. Compressing air like that raises the temperature (just think of how your bicycle pump heats up with vigorous use). If you inject diesel fuel at the top of the piston stroke, the high temperature is enough to cause it to ignite. No sparkplugs required.

That combustion forces the piston out to its extended position. After that the piston comes back to eject the resulting exhaust, extends again to take in air, and and the cycle resumes. The piston is connected to a crankshaft, which translates the back-and-forth motion to a rotational motion that your round wheels can understand.

Diesel engines are strong, and have long lifetimes. Until recently they have been used mostly for very large things, like Mack trucks that need to haul serious loads, or even heavier trains.

Diesel's problems: they are hard to start, and once they get going they are loud, and throbbing, and then there's that black smelly cloud that they tend to belch. While I have always wanted to drive a trailer truck across a continent (almost any continent will do, I'm not picky), I wouldn't want to have a brute of an engine like that on my driveway.

So why does this blog drool so persistently over diesel-powered cars?

Because what's under the hood of a diesel-powered passenger car in the 21st century is a far cry from the monsters that my dad used to wrestle. (Literally. He used to say, snuffing my trucking dreams, that I didn't have the upper body strength to make a truck go around the corner. He was right back then, but driving a diesel car now is no longer a wrestling match, thanks to power steering). Also, the fuel itself has become cleaner.

"Clean" diesel.
Wait: Isn't diesel what's left of petroleum after you take out the gasoline, the kerosene, and the stuff from which shampoo, fertilisers, and all manner of plastics are made? How can the rest possibly be "clean"?

It's still diesel. It still contains benzene, toluene and other organic compounds that aren't good for you. But when most of the sulphur has been taken out, to a level of 15ppm or lower, you can call it "Ultra-low-sulphur diesel" or ULSD for short. And because nobody can remember the acronym, or might confuse it with either a university or a psycho-active drug (and because it sounds so green), it's known as "clean" diesel.

It is actually cleaner than what came before, particularly if you consider that the previous diesel standard allowed for 500ppm of sulphur. At that concentration the sulphur tended to give much higher levels of particulate matter pollution. It also poisoned nitrogen oxide catalysts that were trying to keep smog formation down. Think about it: since the introduction of "clean" diesel in 2007 you don't often see trucks surrounded by a dark cloud of exhaust any more.

That diesel exhaust cloud was black because it was full of soot: mostly carbon that didn't get burnt in the engine. I bet if you could collect those soot particles and compress them into a long thin cylinder you could make a nice pencil.

This is because a piston by itself is not all that good at air intake: at the expansion stage the pressure inside the piston is lower than that of the outside air, and there is simply not enough air to completely burn the fuel injected into the piston.

A turbocharger installed in the air intake presents the piston with air that's pre-compressed to nearly twice atmospheric pressure. That means that you can burn just about all the fuel injected, eliminating nearly all soot in the exhaust. Moreover, the piston stroke is more powerful, boosting the fuel efficiency. A further benefit is the huge reduction of organic hydrocarbons, some better known as volatile organic compounds, or VOCs, that have been implicated in several serious health issues.

Turbochargers, originally developed for airplane engines that had a hard time at high altitudes where air is thin, have come a long way. They are now much more robustly built than their forebears.

Not only that, they get their power from the engine's exhaust gas, so they really pack a large increase in total engine power for very little extra weight. Or rather, they give a significant boost to the fuel efficiency for a given engine power.

Soot filters.
Any soot particles that are still left in the exhaust stream are caught in a particulate trap, and burnt off periodically to regenerate the filter.

Between turbocharging at the front end of the engine, and soot filtering at the back end, the particulate matter emerging from a diesel car's exhaust is lower than that of a gasoline-driven car of similar power. This is particularly true in an aggressive driving style where gasoline engines burn much less efficiently and emit a high level of particulate matter.


As an example, let's look at the Audi A4 Quattro, available in the US with a 3.0L TFSI (turbo fuel stratified injection) engine running on super unleaded gasoline and putting out 269 hp power. The diesel version with comparable power would be the 3.0L TDI (turbocharged direct injection) with 243 hp power, 10% less than the TFSI engine.

Both engines are turbocharged. But the diesel engine gets 50% higher fuel efficiency, 33mpg. This is thanks to the higher energy content of the fuel (15% higher in diesel than in gasoline), as well as the higher compression ratio of the pistons. The resulting lower carbon dioxide emissions earn it a European "B" label whereas the gasoline engine is only good for a "D".

The table shows the German pricing (in the US, the 3.0L TFSI starts at $ 47,300; the 3.0L TDI is not available here). In Germany, as in the US, the net price of the diesel is higher than that of the gasoline version of the A4, but the structure of the German car sales tax encourages the purchase of the version with lower CO2 emission, so the total price, shown in the table, ends up being lower for the diesel.


Audi A4 Quattro, gas vs. diesel.

3.0 TFSI quattro

3.0 TDI quattro
Trim "Attraction" "Attraction"
Year 2012 2012
Emissions rating EURO5 "D" EURO5 "B"
MSRP in Germany € 44,300
($ 59,400)
€ 42,900
($ 57,500)
Fuel Economy:
City/Hwy quoted 22 / 36 mpg 33 / 48 mpg
Avg. quoted 29 mpg 41 mpg
Actual avg. 21 mpg 33 mpg
CO2 emission quoted 190 g/km 152 g/km

3.0L TFSI V6

3.0L TDI V6
Power 269 hp
@ 4780-6500 rpm
243 hp
@ 4000-4500 rpm
Gears 7-spd S tronic 6spd man
Fuel super unleaded "clean" diesel
Emissions for 2011 models:    
Engine 3.2L V6 quattro
262 hp
3.0L V6 quattro
237 hp TDI
Year 2011 2011
Noise 74 dB 74 dB
CO2 214 g/km 173 g/km
CO 0.314 g/km 0.239 g/km
Hydrocarbons 0.043 g/km n/a
NOx 0.036 g/km 0.159 g/km
Hydrocarbons+NOx n/a 0.194 g/km
Particulates n/a 0.5 g/km

I wasn't able to find emissions data for the 2012 models, so the lower half of the table shows data for 2011 models with comparable power, close to 250 hp. UK government data in the table is listed at car-emissions.com.

On most emissions standards, the diesel is actually cleaner than the gasoline version. The only exception is for NOx emissions, which contributes to smog, acid rain and respiratory problems, and which is more than four times higher for the diesel. The hopeful spin on this is that just a decade ago diesel engines, when compared to gasoline engines of similar power, had ten times as much NOx in their exhaust: so there is improvement.

But how does the drive feel?
Last summer in England we ended up driving a diesel rental, a BMW 118d M sport 5-door hatchback. We cursed just about every time we got in, and it wasn't because the steering wheel was on the other side. The seats felt wrong. The dashboard had the wrong curvature even for my short legs. There were large blind spots. CelloDad couldn't get in an out of that car without taking out a piece of skin or a button off the dashboard or something. The trunk looked okay but was full of useless nooks and crannies, so the storage was inefficient. There's no way a cello would fit there.

But the engine! Allright, so it's a BMW engine. To be precise, it was a 2.0L, 143 hp turbodiesel job. It might as well have been a regular gasoline engine. You couldn't hear the difference. You coudn't smell the difference. You couldn't feel the difference (it did throb, but it was a discreet kind of throb). In short, in no way did this car behave like a Mack truck. Especially once you take it on the motorway, where the hum was a pleasure. Anywhere really, it was hard to keep it from zipping around, even with the shift stick in my left hand.

Best of all? We stopped at the pump only once, at the end of a two-week trip. It had done 44 mpg.



You may also like:
1. Have your Cake and Drive it Too: of Fuel Economy, Performance, and Moms
2. Just how bad is diesel exhaust?



  1. Hello, thanks for a nice article.

    I would like to point out few things - Diesel vehicles have their own set of problems. When new, all is fine, but after awhile, they tend to get very problematic because engine and engine components tend to get soothy. To clean the sooth, one needs to press diesel car a lot. Also, engines pretty much want full-synthetic oil. Give it a low quality oil, and it won't work that nicely, and it will release clouds of sooth when pressed.

    I am from EU; and in cities it can get very nasty. Diesels sting disgustingly. It might not look like that in US where there isn't much of them, but here most cars are diesels and collectively they produce large amounts of stink. Cities stink, and if you will ever travel through some of EU tunnels, you will be perplexed by the sooth fog there created by diesels.

    If you ask me, gas is the way to go, and if you want to drive cheaply, do a LPG conversion.

    1. Thanks for your perspective!
      It would be great to switch to natural gas once the infrastructure is in place: right now in the US it's not a practical option.

      I agree with you that particulate pollution is diesel's biggest problem, but I must say that I haven't noticed overwhelming diesel fumes in cities in north west Europe: Germany, Benelux, France, UK - not even in Rotterdam at rush hour, not even in its tunnels under the various waterways. Cities used to be very bad in France up to the 1990s when they didn't have too many catalytic converters, but the situation has improved markedly since then.

      Indeed, there is a recent study done in Los Angeles that shows that gasoline engines are responsible for much more small-particle pollution than diesel engines: http://www.esrl.noaa.gov/csd/news/2012/115_0309.html

      But I will keep your advice in mind to keep my diesel engine clean and not skimp on the synthetic oil it requires!

  2. That's not a Mack in the picture, that is a Kenworth.

    1. I stand corrected! Actually, I don't know the first thing about trucks, much as I would love to drive one.

  3. Diesel engines are more fuel efficient in that you can drive them more miles on one gallon of Diesel fuel vs. a comparable gasoline engine. However, Diesel engines produce more CO2 from burning one gallon of Diesel than a comparable gasoline engine. So, if you look at miles-per-gallon Diesel engines are considerably better than gasoline engines; however, if you look at CO2-per-gallon, Diesel engines closer to gasoline engines (still better, but closer).

    Let's look at both Miles/Gallon (MPG) and CO2 Grams/Mile (CPM) from http://www.fueleconomy.gov/ I tried to compare "equivalent" engines as much as possible i.e. roughly comparable engine volume and horse-power, on the same exact model car.

    2014 Audi Q5:
    * 3.0 TDI = 27 MPG, 385 CPM
    * 3.0 Gasoline = 21 MPG, 429 CPM

    The TDI model produces 385*27 = 10.4 Kg of CO2 from burning 1 gallon of Diesel, while the gasoline model produces 21*429 = 9.01 Kg of CO2 from burning 1 gallon of gasoline.

    This means that 1 gallon of Diesel gets you 29% farther than 1 gallon of gas (=27/21), but produces 15% more CO2 than 1 gallon of gas (10.4/9.01).

    2014 VW Golf:
    * 2.0 TDI = 34 MPG, 297 CPM
    * 2.5 Gasoline = 26 MPG, 342 CPM

    1 gallon of Diesel gets you 31% farther, but produces 14% more CO2 vs. 1 gallon of gas.

    2014 VW Jetta SportWagen:
    * 2.0 TDI = 33 MPG, 310 CPM
    * 2.5 Gasoline = 26 MPG, 342 CPM

    1 gallon of Diesel gets you 27% farther, but produces 15% more CO2 vs. 1 gallon of gas.

    2014 VW Passat:
    * 2.0 TDI = 34 MPG, 290 CPM
    * 2.5 Gasoline = 25 MPG, 350 CPM

    1 gallon of Diesel gets you 36% farther, but produces 13% more CO2 vs. 1 gallon of gas.

    2014 Chevrolet Cruze:
    * 2.0 Diesel = 33 MPG, 307 CPM
    * 1.8 Gasoline = 27 MPG, 333 CPM

    1 gallon of Diesel gets you 22% farther, but produces 13% more CO2 vs. 1 gallon of gas.

    Don't get me wrong: I think increasing efficiency and reducing pollution are critical goals, and Diesel can definitely help. I just think that simply measuring relative efficiency through MPG paints an incomplete picture if the actual goal is CO2 reduction.

    Thanks for a great blog!

    1. Thanks for stopping by! You're absolutely right: burning a gallon of diesel produces more CO2 than a gallon of gasoline, by about 15%. But if the efficiency is higher by more than 30%, you do still gain. The real measure, climate-change wise, is pounds of CO2 per mile (or g CO2 /km, as used in Europe). Miles per gallon is probably the worst measure in many ways - at the least it should be gallons per mile.

      But the REAL CO2 savings to be had from internal combustion engines, is to start driving cars with smaller engines. This is where diesel engines can make a difference: we could start driving cars with much less horsepower under the hood (the ones we have now have twice as much as we need), but that still have a kick.

  4. I agree that CO2/mile or, at least, gallons/mile would be better than miles/gallon for measuring vehicle efficiency. That being said, most people are interested in (and aware of) cost as measured in dollars spent on gas. The "cost" of emitting 1 pound of CO2 into the atmosphere is, effectively, perceived as zero.

    There are many factors that affect the emission of CO2 from driving, such as:

    1. Fuel consumption
    2. Kind of fuel (regular vs. Diesel)
    3. Engine power
    4. Engine capacity
    5. Vehicle weight
    6. Vehicle drag coefficient
    7. Mix of highway vs. city driving
    8. Personal driving style

    I don't know enough to tell whether (unnecessary) engine power contributes more towards the final CO2 emissions than, say, the drag coefficient or the mix of highway vs. city driving.

    At the end of the day, driving a car -- any car -- for a year will produce X metric tons of CO2. It might be simpler and more actionable to simply tax or somehow offset these CO2 emissions directly (and set this up in a way that's not regressive).

    1. For a given model car, installing a smaller engine makes a huge difference, up to 50%, in the carbon emissions. Many of us can't escape the reality of the inherited infrastructure (suburbs), but we can try to make a difference by choosing our engines smartly. Smaller engines are also cheaper to buy, to me a more attractive option than curbing my emissions by buying a more expensive hybrid.

      A carbon tax is politically very, very difficult to introduce. But where it's in place, it's extremely effective; see the Dutch example http://www.cellomomcars.com/2013/03/carbon-tax-is-effective-tool-to-reduce.html


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