Old diesel technology: In 1992, as a poor student, I bought my first diesel car - a manual shift 84’ Toyota Camry, not many people know that Toyota had made diesel sedan. I got it from a friend for $800 and almost 100K miles on it. It was in upstate New York where it’s really cold and a lot of snow in winter. When I got this 8 year old light color car, the body was pretty much brownish as a result of the rust. The salt has taken a huge toll on the body of Japanese car. At that time, people felt the biggest advantage of having American car was the body integrity other than safety. But this diesel engine ran great. It could get 34 mpg on highway while normally Honda civic could get 26mpg then. There were a few times the battery died on me, but I was able to push start it. This is one big advantage with diesel car over gas car. Gas car can be pushed start on the condition that battery is weak but not totally died. That diesel car didn’t have spark plug since the ignition is due to high pressure rather than spark ignition. Maintenance was simple (still need oil change) and all done by myself. I put on 30+k miles in two years until finally it died and I had it towed to junk yard since fixing it would cost more than it’s worth. Nowadays, many diesel engines may have spark plugs for more efficient burning plus so much electronics usage and I have no idea if we can still push start if the battery is out. As for the fuel station for diesel, occasionally I did encounter difficulty in finding it in an unfamiliar place. But always plan ahead instead of starting to look for stations with last 10 miles left. Gas stations usually have diesel price indicated at the bottom of gas prices sign. So it’s easy to spot from distance. The diesel Camry had a turbohcharge which kicks in when the rpm is high.
Turbo technologies - turbocharge vs. supercharge. Turbocharge recycles energy from exhaust and generate compressed air which is then fed into engine cylinder for more efficient diesel burning. Since the turbocharge is powered by the exhaust gas, so it won’t come on unless engine reaches high rpm. This is the reason for so called “turbo lag”. So turbocharged engine may feel a bit sluggish if start from zero when the turbo is not on. Especially if you punch the gas pedal hard, there is 1~2 seconds when you feel the engine is not producing the power you are expecting. The easy way to cope with this kind of “lag” feeling is by pushing the gas pedal gently. There is still sufficient power for uphill acceleration but not as brisk as gasoline engine. Once you are on the highway, then you feel like a champ for easy passing. The high torque means the engine cruise at lower rpm which will cause less engine wear. In X5 case, at 80mph the engine is around 2200rpm while normal gasoline engine will be at 3000 or more. So for long term keep, diesel car is the way to go.
Another kind of turbo technology is called supercharge turbo which is powered by the engine (tied in to engine shaft by belt). The engine power will turn the turbo mechanics and produce compressed air. Supercharge turbo is more used in race car since it’s less fuel efficient but provide more power at low rpm. You may Google to find out more if interested.
Diesel fuel and gas mileage: Diesel price is normally between premium and regular gas price. But recent few weeks, it seems to be higher than premium in my area. People are normally concerned about diesel car in cold weather since the fuel may clump and won’t be able to start. But it is normally equipped with heating system to prevent it. My old 84 diesel Camry had this function already. So in really cold weather, I had to turn on the switch for half or one minute waiting before starting the engine. But it never failed on me. I suppose newer cars with latest technologies should be doing better and no need for the wait in cold weather. The drawback with diesel engine is that the power might be a little weak from zero start. But for normal driving, I find it sufficient even for uphill acceleration. But if you always want to be the first coming out of the pack when lights turn green, it’s better to get a gas version. But it’s much more fuel efficient and plenty power for highway passing when turbo kicks in. So far my X5 is averaging 24mpg for the 10+k mixed driving miles (~60~70% highway). Purely local driving will be around 20~21mpg. I thought this is really decent for 5400pound car. Gas version Acura MDX which is 400pounds lighter than X5 is probably around 16mpg for local driving. For other lighter diesel sedans, the gas mileage should be easily getting 40mpg or more. So if the car price is similar, no question diesel car is more cost efficient than a gas car in the long run. Prius has excellent fuel efficiency, but the price tag has offset that cost advantage. For the same Prius price, you may get a larger car for peace of mind.
Diesel fuel quality: should use Ultra Low Sulfur Diesel (ULSD) type which is mandated by the law. But in some countryside, lower quality diesel is supplied for farming purpose diesel engines (such as tractor I suppose). So you need to watch out ULSD sign on the pump in the countryside. In California, it seems that some stations don’t put out that sign and I suppose it’s by default ULSD grade. Also, centane level is an indicator for quality of diesel (http://en.wikipedia.org/wiki/Cetane_number). Premium grade diesel is supposed to make the engine run much quieter. But I have never been able to find “Premium” sign on pump, but after trying several stations diesel, I found one of them seems to have diesel which makes engine seemingly quieter (or maybe it’s in my mind since I can’t tell for sure the difference). If you find the diesel engine becomes noisy, likely the poor quality diesel (low centane) fuel is being used. The characteristic diesel engine knocks can become pretty loud. So look out for more reputable gas stations.
DEF (Diesel exhaust fluid): it’s 32% urea in water solution and has a shelf life for 1 year. The freezing point is -7degree C. Normally there is a heating mechanism to keep it from frozen as soon as the car is running. The function of using DEF is to assist catalytic converter to decompose NOx gas into N2 and O2 for cleaner burn. In recent years diesel trucks or SUVs all have equipped with a tank holding DEF. I believe this is part of the cleaner diesel technologies. Using my 2011 X5 as an example, it has run 10+k miles now and DEF is supposed to be replenished when it returns to dealer for every 12K mile maintenance (oil change, filter change, DEF replenish, etc. which is all free of charge for the first 4 year). BMW uses synthetic engine oil which only needs to be changed every 12K miles. Some perfectionists would change engine oil or replenish DEF by themselves way before hitting the first 12K miles. Occasionally, BMW dealer forgot to fill up the DEF tank before the car is delivered to customer (there are two tanks, the smaller one is filled when the car is out of factory but the dealer has to fill up the larger one). If that happens, the car will give a warning light after few hundred miles. Once this warning light is on, you can only drive X5 for another 1000 miles before it completely disable the engine. This means that you have to fill it up soon either by dealer or yourself. Some people prefer replenish by themselves rather than driving to the dealer. DEF replenish can be DIY easily and cheap (check this link for X5 example: http://www.bimmerfest.com/forums/showthread.php?t=459473&highlight=def).
So my conclusion for diesel car is that it represents a new fuel efficient technology and we should not be scared of it simply because we don’t know it well. Since US is pushing for cleaner air technologies, there are a lot of government incentives if you purchase diesel cars. Many manufacturers, such as BMW, offer $3500 diesel credit for X5 and $1800 from federal tax return. With all these things factored in, the diesel version price is about the same as gas version. So we should take advantage of that. Like some friend pointed out, diesel car resale value is better than the gas version due to the better fuel efficiency and longer engine lifetime.
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