Toyota Prius vs BMW diesel
March 27, 2008 by Abhishek
In an interesting experiment, two Times correspondents drove from London to Geneva. One of the cars was a Toyota Prius hybrid while the other was a BMW 520D.
The results?
The Prius averaged 40 mpg (48.1 miles per imperial gallon) over the whole journey while the BMW achieved 41.9 mpg (50.3 miles per imperial gallon).
Admittedly, the test wasn’t completely fair. Prius’ strong point is city mileage and the vast majority of the journey was along a fast highway where they drove at 78 miles an hour with a strong headwind. Besides, the BMW ran on diesel which always gives better mileage than petrol (but also emits more CO2). Still, I think it was an interesting experiment and showed that at least some of the claims about the Prius are overstated.
I looked to see how much difference there was between diesel and gasoline in CO2 emissions. Everything I saw said it was the other way over. They were saying that diesel gets better mileage AND emits less CO2.
That left me puzzled. I guess that black smoke and the funny smell I get when caught behind a big diesel bus or truck is just unoxidized carbon? I’m guessing that the total emission of stuff from diesels used to be small enough that it was overlooked because there were so few of them, but that is changing (something like 40% of new cars in Europe are diesel). Anybody know the overall nature of diesel output?
And I don’t think it’s very informative to have performed this test using 80 mph driving. The eventuality is that we’re all just going to have to do less of this kind of long distance driving. When gas is $10 or $20/gallon, we just won’t pop the family into the car and drive 500 miles. So speeds that high won’t be so important for the shorter kind of driving that will survive.
But even so… why can BMW make a 42 mpg 4 cylinder diesel, Toyota can make a 40 mpg hybrid (44 with more reasonable driving), and Detroit seems to be busting a gut to get to 35 mpg? We always seem to win the battle for silly things like video tape formats or high density DVD formats, but when it comes to something like saving the air breathe, corporate America seems clueless.
I also noticed the error about the CO2 difference between diesel and petrol (gasoline) engines. Bob Hickling is correct, diesel engines not only get 30-40 percent better mpg than a gasoline engine of the same performance, but produce about 30 percent less CO2.
A hybrid gasoline vehicle can produce less CO2 than a non-hybrid diesel in city driving, but not at higher speeds. The most efficient and environmentally sound solution, other than a totally electric or a fuel cell vehicle, would be a diesel hybrid. Unfortunately these would be very costly.
The higher mpg of a diesel vehicle is partially because the fuel has a higher energy content per volume than gasoline, and mainly because diesel engines use higher compression which is more efficient in combustion. The diesels can offer another advantage if they are able to run on bio-diesel.
To compare performance between a gasoline and diesel engine, you must use vehicles of the same weight, and compare acceleration. For the same performance, diesel engines will produce less horsepower, but much more torque and produce it at a lower rpm. To use this torque a diesel engine must be coupled to a Constantly Variable Transmission (CVT) or a transmission with more gears and use a differential with a higher gear ratio than a gasoline engine.
Diesels in North America are widely used for commercial, farm, and construction vehicles where their torque and fuel economy are needed. There were some popular diesel passenger vehicles sold here in the 80’s, but all of them were imports. The domestic auto manufacturers made some poor attempts to produce diesel passenger vehicles at that time using existing parts from gasoline vehicles. They had very poor performance and reliability. Many people have not forgotten this. If they had used parts designed for this purpose, and added a turbocharger for performance, this would have been avoided.
Volkswagen and Mercedes have been the main companies selling diesel cars here since then, and diesels have accounted for 50 percent of Volkswagen’s sales in some areas because the TDI’s got close to 50 mpg.
When the U.S. switched to Ultra Low Sulfur Diesel (ULSD) in October of 2006, there were some very strict emission standards that also went in to effect. The emissions must meet those for gasoline engines, and the exhaust system must last for the lifetime of the vehicle. This is much stricter than even European standards. It requires the exhaust system to be made of very expensive stainless steel. This meant that even the modern Common rail Direct Injection (CDI) diesels that have been sold world-wide for years now did not meet the U.S. requirements.
For those that are not familiar with modern CDI diesels, they use a higher injection pressure and computer controlled piezo-electric injectors that give multiple small bursts of fuel to a cylinder per combustion. This almost eliminates any knock associated with old diesel engines, gives better fuel efficiency, reduces emissions, and increases performance. They also use at least one turbocharger if not two, (a small one to spin up fast and give boost off the line, and a larger one to give boost at speed). The computer control also allows changing from a fuel efficient mode to a sport mode with a switch. They also use catalytic converters and other methods to reduce emissions. There is no big cloud of soot anymore.
So far, Mercedes has been the only company selling diesel passenger cars in the U.S. since the switch to ULSD. Volkswagen will start selling the Jetta Sportwagen in October 2008, which will get 45 mpg (US) City and 55 mpg (US) highway. Honda will follow by replacing the Accord Hybrid with a 52 mpg (US) Highway? diesel in the first quarter of 2009. Audi and BMW will also be offering cars in 2009.
Ford and GM have no plans to sell any of the diesel cars they sell to the rest of the World in North America. I have been asking them for years now, and would probably buy a Ford Mondeo diesel if they offered it.
I won’t place blame on anyone, but it is not totally with the North American consumer. Many are not aware of what Ford, GM, and other companies sell in the rest of the world. They have not been given the options available elsewhere.
I know there are many military people like me and others who have lived overseas that keep asking for diesels. We keep getting the same bullshit from the PR people at Ford and GM. They say there is no market for them here except for commercial vehicles. Then they charge $5,000 for that option.
These companies have been losing market share in North America for years because they would not even install variable valve timing in engines that were designed to use it. I have no sympathy for greed or stupidity in an otherwise intelligent people, ignorance is another thing entirely and can be overcome.
P.S.
I forgot to mention that Ford, GM and most auto manufacturers only charge between $0-1,200 for the diesel option in other countries. The price for the diesel option on imports here in the U.S. is still consistent with the price in other countries.
GM and Ford should pay attention!!!