Diesel vs Petrol motorcars. Pro's and cons?

Question

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.

Answer 1

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 engines in cars are currently looking very good.

Answer 2

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!

Answer 3

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...

Answer 4

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.

Answer 5

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.

Answer 6

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.

Answer 7

As it happens with many such questions, the answer is "it depends".

Diesel engines are more expensive. In long term, diesel engine car is cost-efficient (taking into account purchase price and fuel costs) when you are making greater mileage per year. It should be easy to calculate savings on fuel vs higher purchase price of diesel-equipped vehicle. Conclusion: if I need a car to driving to office a few kilometers per day (happens if you live in Europe and work in your hometown), petrol may be better choice.

Modern diesel engines are sophisticated machinery. Good old rule of thumb says if there is more complexity, there are more reasons to break. Especially when buying used car, which mechanical warranty is already expired, you may want to keep in mind that:

• Dual Mass Flywheels, as suggested by @Chris McKeown are not going to live forever. Often they are not repairable, and expensive to replace.
• Diesel Particulate Filters (DPF) are installed in most (if not all) modern diesel engines to reduce soot from exhaust gases. Unfortunately, short driving cycles, when engine is just barely heated up, will lead to jamming of the filter. Onboard computers sometimes order "burning" process by enabling all possible electrical devices in car, increasing exhaust temperature (and fuel consumption). If that not helps, the filter needs to be replaced, which may be expensive. If you driving mostly in your hometown, risk of failure is substantial.
• Common Rail fuel feeding is powered by high pressure fuel pump (up to 2000 bars), and precise piezoelectric injectors. This equipment is operating in extreme conditions, which increases rick of malfunction, especially over long time.

Bottom line: diesel engines are generally more efficient, have more torque, they do not sound so bad nowadays :), but it comes with a cost of complexity and higher price. It may be less a problem when purchasing a brand new car from dealership and have all issues covered by warranty.

Petrol engines are generally considered "safer" to maintain, but not so fuel-efficient.

Answer 8

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..

Answer 9

As far as I know is Petrol better than Diesel because of that:

The issues with fuel

Diesel is not the clearest fuel in the world. Of course, it depends at first, from which petrol station you are getting your diesel from. Like the Jet station or the Roth one, some of them sell unclean petrol or diesel. So, that the mixture does not fit with the regulations from the European union. Or they get rid of old, -unclean, -dirty petrol rather diesel. And of course, this fuel is cheap, cheaper than getting good fuel from OMV or bp, where you could be sure that your fuel is more clean. But if you filling in old or dirty diesel rather petrol your emissions are higher rated and your particle filter suffer from this, so maybe it blocks up the fuel and your engine stops. To mend those issues, you need more money to fix it than to buy a high-quality propellant. I can understand, if your car or rather your engine is thirsty, the fuel should be quite cheap. But think on the health of your car and the health of your lungs.

Money money money

The next point is of course the money. Money is a very important point on this comparison. Diesel is cheaper than Petrol but it´s more difficult to inflame, so the piston gets less power for more horses. The point is of course that diesel needs less fuel but more power from the batteries. We must see it in this way: All parts from the diesel engines are more expensive, for example the engine needs more power to fire it up because of the diesel; its, as I said, less flammable and that’s the reason it needs glow plugs. Glow plugs are for diesel engines and if you have a cold engine you need a big amount of electricity for the preheat. It is heating up until 55-75 degrees. Diesel is only flammable when the glow plug is in that temperature that the diesel can vaporize. This steam is now flammable plus the diesel which had not vaporized yet in the inside of the cylinder. So, a Diesel engines has got less rounds per minute and it´s not as thirsty as a Petrol car because the vaporized diesel do not need a big amount of fuel to fire up. So, the alternator must be more powerful and the battery bigger. All in all, is diesel cheaper than petrol but it´s “producing” a big amount of bad emissions and your lungs suffer from this.

Answer 10

For every day use diesel are better:

• diesel cost less (in EU)
• most modern diesel engines are near to petrol engines as performance
• diesel engine lasts twice of a petrol engine (ex. petrol makes about 250'000k, dieasel 500'000km
• petrol engine are simpler (less components) then diesel, so less problems
• petrol engine is economic only if combined with gas (Metan or LGP)