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I realize this may be a silly question to those of you on here who are proper petrol-heads and this may even be a repeat question, but I could not find anything on it on here, so here goes:

What are the advantages and disadvantages between cars with petrol engines vs those with diesel engines?

I've heard people say things like diesel engines produce more raw power, but are slower to get there. As far as I know, diesel engines need to warm up the fuel using the glowlamps (I think that's what they're called) before ignition is possible. When I was a kid (grew up in South Africa) only the pickup trucks were diesel-powered and diesel used to be way cheaper back then.

I'm not interested in the inner workings of petrol vs. diesel engines, but more in what would be of relevance to an everyday buyer when choosing between diesel vs. petrol. I realize this question might be a bit broad, but I'm looking for differences one can expect (in general) between modern petrol- and diesel-powered motor cars.

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I'm not interested in the inner workings of petrol vs. diesel engines, but more in what would be of relevance to an everyday buyer when choosing between diesel vs. petrol I'm not sure this is about motor vehicle repair. This seems more like a shopping question, which would be off-topic here. –  jmort253 May 15 '11 at 4:35
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7 Answers

up vote 8 down vote accepted

As an everyday buyer you won't be interested in the high performance end of diesel or petrol, so here goes:

  • Diesel engines typically rev lower and have more low speed torque - so you are less likely to stall off the line, for example.
  • Petrol engines usually rev higher and get more power at 3-4 thousand revs
  • Diesel seems to be getting more expensive - especially in the US.
  • Diesel engines often run more efficiently - more miles per gallon (sometimes significantly more)
  • Diesel engines are noisier, although manufacturers are improving all the time.
  • In the UK you pay less emissions taxes on diesel engines, as the tax is heavily based on CO2, not particulates.

So from a pure running cost perspective, diesel engined cars are currently looking very good.

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it's worth noting that the noise of diesel is sometimes moderated by the turbo (if they have one) acting as an additional muffling device. The noise is quite different from a petrol car, though: I've always thought that a diesel engine sounded like a tin can half full of marbles falling down a flight of stairs. Economical sense: yes. Aesthetic appeal to me personally: not so much. –  Bob Cross May 11 '11 at 16:40
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@Bob - totally with you on this. And the Golf tdi is a very well behaved car... but I do like my cars to have at least 350 bhp and be made by Subaru:-) –  Rory Alsop May 11 '11 at 17:08
    
and @Dude, thanks for the answers, very helpful. I chose Rory's as the accepted answer since it's a little more detailed. Appreciate it, guys. I recently came into possession of an Alfa Romeo MiTo (1.4l Turbo, MultiAir) it's amazing what they've managed to get out of such a little engine. As a now-Alfa owner I'm trying very hard to upgrade my petrol-headedness. :-) –  DeVil May 12 '11 at 6:47
    
Definitely not cheaper here in the states. Diesel costs significantly more than gasoline most of the time. Exceptions being during the big gas price spikes, when diesel is comparably priced (per gallon, per mile cost is of course a completely different result!). –  Brian Knoblauch Jul 17 '12 at 11:37
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Another downside I just thought of. Diesel engines are MUCH harder to re-fire if you run them out of fuel. Which happened to me once because we couldn't find any fuel stations that served up diesel and ended up running out trying to find one... The re-priming process was inconvenient, time-consuming, and we still nearly killed the battery by the time we finally got it restarted after someone brought us some fuel... –  Brian Knoblauch Dec 5 '12 at 17:13
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Another difference is that the diesel engines cost more to produce and to maintain. So the diesel version is almost certainly more expensive to buy than the equivalent petrol car, and it usually costs a bit more in servicing. So the rule of the thumb is to buy diesel if you're going to drive more than ~10000km per year or so, depending on fuel costs, taxes etc...

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As the owner of 5 diesel vehicles, I will agree that the purchase price is typically more than a gasoline equivalent, but I don't agree that maintenance is more expensive. In fact, it is often cheaper. Although, that may very well depend on your locality. –  Flimzy Aug 27 '11 at 9:04
    
Yes, I would like Engin to elaborate as well on his point of more expensive maintenance maintenance. –  theUg Jun 27 '12 at 22:33
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This very much depends on where you are but for Germany you can find the cost estimates for all actual cars here: fuel-pilot.de I guess the numbers on that site would apply as a rule of thumb in Europe but I have no clue how they compare to the US market. –  Engin Kurutepe Jun 28 '12 at 14:53
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In short, diesel engines tend to be more reliable and more robust. They're also far more fuel efficient than their petrol counterparts.

Also, diesel engines may be down on power but what they lose in power they make up for in torque. Torque is more noticeable in an urban environment because of the stop and go nature of traffic. You need quick acceleration of the line and diesel's are great for this...Look at trucks/lorries, they need lots of torque to get off of the line when carrying heavy loads.

However, there is a premium that you pay over the diesel cars though, at least here in the States. Diesel is also more expensive because the of the new ultra-low sulfur content laws. BUT they certainly make up for in efficiency!

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Ahh - your diesel is more expensive? For some reason I assumed it would be lower there as well. Will update my answer. –  Rory Alsop May 11 '11 at 17:09
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Yeh, its about 10 to 20 cents more expensive. Checking the local prices though I see that they're about the same. There are times when they fluctuate but diesel never dips below petrol here anymore. –  Dude318is May 11 '11 at 17:24
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In Europe, diesel has been at least 10-20% cheaper since forever but in the past 2-3 years the prices are nearly identical. I've only once seen diesel to be more expensive than petrol in Austria; never more expensive in Denmark - just to provide two points of data. –  Torben Gundtofte-Bruun Jun 11 '11 at 19:06
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You should look for subsidies in your country for diesel, that might make owning an diesel vehicle (and the related extra maintenance costs it brings with it) worthwhile. In India, we usually get a number around 100kms / per day. If your daily commute is greater than that, diesel is economical.

Running comforts: you should know that you can't (or shouldn't) rev your diesels to max RPM - because most diesel engines provide max power at mid RPM. So by doing this - all you are doing is heating up the engine and damaging it in long runs. So essentially, if you are going for high-revving experience / sound, diesels are not your choice. They make quite a bit of noise as well, so if your ideal driving experience involves a silent ride, diesels don't cut it either.

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A point I haven't seen mentioned here: diesels will last a lot longer -- often twice the lifetime or more of a gas engine.

That might not seem to matter if you "flip" vehicles every few years, but because of that lifetime, even used diesels tend to have higher resale value -- sometimes MUCH higher! I've seen a Dodge 250 gas pickup for $500, but the Cummins diesel version in the same year will fetch $5,000, because the engine hasn't even hit half its life yet. Some diesels should go a million miles with only a couple expensive maintenance items, typically, rebuilding the injection pump every 300,000 or so.

(Of course, the brakes, electrical, and other running gear ages the same on a diesel as with gas.)

The fuel economy is often dramatically greater. A mid-80's diesel Jetta will get the same highway fuel economy as a new Prius! My 1991 Dodge Cummins gets 20 mpg, while the gas version gets 12. My diesel step-van gets 14, the gas step van I replaced got 7. If, as I do, you believe fuel costs are going to continue up and up and up, this may be a huge consideration.

Finally, if you think that biofuels are the future, consider that a diesel running on biodiesel will perform within 5% or so as well as it will on petro-diesel. But a gas vehicle running on ethanol will lose about 30% of its power and fuel economy.

I make my own biodiesel from restaurant waste oil -- it's relatively easy, compared to making ethanol.

I'm also of the opinion that maintenance is cheaper than that of gas engines. Diesels are MUCH simpler than spark-ignition engines: it either isn't getting fuel, or it isn't getting air. The heart of a diesel is the injection pump, and those are pricey to service or replace. So you tend to have low maintenance costs until you need a pump, then a big bill every few hundred thousand or so. But over the lifetime, I'd have to say diesels have lower maintenance costs.

But the real question here is, do you consider yourself mainstream, or a bit eccentric? If you like the "edge," and are trying new and different things out, go for the diesel. But if you cringe and being out of the middle of the bell-curve, gasoline is best for you.

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I work on diesel engines daily and totally love them. It's the lack of knowledge about diesel engines that makes people believe they are more expensive to maintain, repair or service...they arent..you dont have to replace plugs every time you service like you would on a petrol car and you dont have problems with plugs misfiring or distributors giving problems.

All you have to do to service a diesel vehicle is change the oil and fuel filters and your diesel vehicle will be running like brand new again. It gives you more toque at lower rpm extending the life span of the engine itself as the rule is the more torque an engine has got the longer the life of it as you dont have to rev it up high for it to perform at its peak. Ooverall diesel does give you better service, reliability and is cheaper to maintain..

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You may have some misunderstandings about petrol engines - I certainly don't change my plugs every service :-) Other than that I think you are broadly correct. –  Rory Alsop Jul 17 '12 at 11:07
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Something that hasn't been mentioned is that a lot of modern diesel vehicles have Dual Mass Flywheels to even out the power delivery. Unfortunately these DMFs seem to be the Achilles heel of many models, failing at relatively low mileages and being expensive to replace.

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