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One of my reasons for buying a diesel was environmental -- I thought it would damage the planet less. It seems I was mistaken. But could a diesel engine be converted to petrol (gasoline)? Obviously this is not a home project, but as far as I understand it, for a reasonably modern (post-2000) diesel you would need:

  • new cylinder head (spark plugs and lower compression)
  • new wiring for the spark generation and distribution
  • new software in the engine control systems
  • (maybe) different cam profiles (not too tricky if you're fitting a new head)
  • You might need different gasket/filter materials in the fuel pumps and injectors

... which doesn't sound like too much. The engine would be overweight and thus sub-optimal, but you would avoid the problem of disposing of millions of old diesel vehicles. In general, are there serious problems in converting diesel to petrol?

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  • Different fuel pump(s) and injectors - pressures are a lot different...
    – Solar Mike
    Commented Mar 9, 2019 at 23:25
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    Realistically, if you want to change from diesel to gasoline, you'd need to either swap the engine (and other running bits) wholesale, or just sell this car and get a different one. There's really no easy way to do it. Commented Mar 9, 2019 at 23:34
  • The petrol engine also needs a different trottle valve and, though technically not necessary, a cat. Modern diesels all have turbos, but those don't have to cope with the very hot exhaust gas of a petrol engine. Remove or exchange it. And what about the gearbox? Diesels run slower than petrols. When you finally changed everything, what's the milage?
    – sweber
    Commented Mar 10, 2019 at 10:17
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    Apart from all the alternatives suggested you may also convert your car to an electric vehicle if you are so prone to keeping it. It may be a better investment if you are consedering something as big as an engine swap. There are some DYI convertion guides online and there are some people really excited about the whole thing. There were also a cult of people running their diesel engines on vegatable oil. It sounds sketchy but it has been around for few years. Maybe there is some hidden genius to the whole thing. (Most likely it is nonsense) Commented Mar 10, 2019 at 18:17
  • @sweber so gas engines have very hot exhaust - and what about diesel? Does slower mean cooler?
    – Solar Mike
    Commented Mar 10, 2019 at 20:22

3 Answers 3

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Biodiesel makes diesel OK

From an environmental POV, you are actually in a better position with a diesel -- if you can find Biodiesel B100 (100%).

You may know that ethanol was found to be a complete boondoggle, as recent studies have shown. However, biodiesel actually does work, because transesterification is a low-energy process (unlike distillation lol, what were they thinking?). So biodiesel is very much a "player" in biofuels, if you can get it.

Diesel to petrol

However, if you want to do a petrol to gas swap, there's a problem. The engine, transmission and smog controls are tightly integrated to each other, and cannot be separated (at least if you want it to work). So you need to obtain a donor vehicle of same model and year, place it right next to the car in question, and swap the entire powertrain and controls. You need the donor car there, because you'll keep finding more and more things which you didn't think needed swapping, but do, and if the donor car has gone to car heaven, you'll be scurrying around at scrapyards trying to find those bits.

You would swap the engine and transmission together, with all smog controls and the ECU still attached. In my swap, I didn't even drain the engine and transmission oil. You may also need to change halfshafts to match the new transmission, but that is routine stuff. Might as well go "new" on those.

Whatever your country does for smog, you'll need to jump through the requisite hoops. In California that is easy if you stayed in the same model and year.

Diesel to electric

As you know, electric has arrived. Charging infrastructure is everywhere, people have recovered from their "range anxiety", and here's the cool part:

There is a huge community of home builders doing "fuel to electric" conversions. And a huge aftermarket supporting them. There is no problem at all getting the necessary kit, including to support IEC 62196 sockets for charging at standard EV charging stations, and sometimes even DC fast charging!

And by using pulled battery modules from wrecks, EVs can be home-converted that have reasonable range, at sane cost. Certainly competitive with an engine swap.

I suspect at this point, "fuel to electric" is actually the most common swap! I myself have done my last "fuel to fuel" swap. I'm American, and we have US-50. I no longer see that as a problem.

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  • Where I am (and I am also American) the most common swap is any other engine to an LS. Oh, and what is US-50?
    – Glen Yates
    Commented Jul 7, 2022 at 18:30
  • @GlenYates well I can't refute your statistic since my swap is an LS. You're right that EV conversions may be a regional phenomenon due to cultural influences... and well, smog testing lol. It gives you a second way to keep your Red Barchetta. US-50 is (among other things) the Loneliest Road in Nevada. In 1992 Noel Perrin (author) planned it with an EV conversion. NvDOT said "Forget it, it's more than 40 miles between AC outlets". But even homebrew EVs now have much better range due to ready availability of EV batteries from wrecks... and NV is installing stations on US-50 every 30 miles. Commented Jul 7, 2022 at 18:53
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It is possible to "convert" an engine from Diesel to Petrol/Gasoline, but the only significant parts that can be reused is the bare block itself.

You'd require new head/s to get a sparkplug hole per cylinder.

You may plan to replace a diesel injector with a petrol injector, but that won't change the compression ratio. Diesels run somewhere between 15:1 and 20:1 whereas petrol is more like 8:1 through 12:1.

Flywheel - maybe? Depends how much it weighs and how that fits the engine's rotational weights.

Alternator - a 24V petrol motor would be weird. A 12V diesel is less-weird but still unusual. You'll probably want to convert from 24V to 12V so every lamp in the vehicle will likely need changing too.


On the plus side, you can probably reuse these parts well enough

  • Fuel tank, once its completely cleaned and purged. May as well reline it now too.
  • Gearbox and clutch, though the ratios might no longer be optimal for petrol. Diesel tends to have more torque at lower RPMs and the gearbox ratios reflect that.
  • Clutch and plates
  • Starter - probably reusable, though many diesels are 24V, and a 24V starter tends to be a bit slow-turning to start on 12V. The opposite is also true - a 12V starter works great at kicking over things on 24V... for a short while.
  • Cooling system - water pump and radiator etc are likely oversized compared to what the same engine doing petrol requires - diesels tend to run hotter because of compression, and are often moving at a lower average speed so less air cooling means bigger fan and radiator.

The main reason for not doing this is liability and financial. A skilled mechanic might make this work after a lot of hours, and the insurance company may refuse to cover it on the percentage of modifications. And if you're paying mechanic's rates for months of work, there will be no monetary gain.

Sell it, buy the vehicle you think is best, be that petrol/gasoline, hydrogen, LPG, more-modern diesel, or the new electric options. Or ride a bicycle, that requires fuel in the form of pies and coffee :)

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Diesel engines are more efficient than petrol engines. Today's diesel-powered cars have to comply with the same emissions standards as petrol-powered cars. Don't let headlines sway your outlook. Volkswagen/Audi put fuel economy and horsepower ahead of nitrosoxide emissions and paid dearly for it. Enviornmental issues today are clouded in politics and the almighty dollar.

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  • No, the emission standards are not the same. Petrol may emit more CO, diesel may emit more NOx. The low emissions and efficiency comes with a cost (on a diesel, at least): diesel engines contain more high-tech parts that are expensive to repair.
    – juhist
    Commented Mar 10, 2019 at 17:50
  • Diesel contains more energy per litre (but not per gram). However, we sell fuel by the litre not by the gram, so it seems significantly more efficient. If you factor that out, diesel is still slightly more efficient, so you're right, but largely because it can't make as much power. Can't waste fuel on jackrabbit starts if your car doesn't do "jackrabbit". Commented Jul 7, 2022 at 0:54

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