Can you change a diesel engine into petrol?

you could, but almost everything would need serious amounts of engineering work.

In practice? No. Be cheaper to source the petrol version fitted to your model and change the entire thing. Even cheaper to just change the car for the same age petrol version.

Depends on type of combustion chamber.

Many Diesels have a separate combustion chamber that communicates with the cylinder by a short passage. This type would require a new cylinder head.

For other Diesels the conversion would require at least a thicker head gasket to reduce compression from 14-19:1 down to 7-9:1. It would require a carburettor or port injection, the diesel injection system can't run petrol as it doesn't lubricate the pump or injectors. The glow plug would have to be replaced by a spark plug and an ignition system / distributor fitted in place of the pump. As the combustion chamber shape would be very poor it would be prone to pinking and be unable to run adequate advance so produce very poor performance compared to a properly designed SI engine.

For many car and light vehicle engines there is a petrol engine that has the same block as the Diesel. But even these may not have parts that can be swapped over, often the bore is different preventing pistons from fitting and the block deck height may not be the same.

Far easier to just sell the vehicle and buy the same car with a SI engine.

Next best option is replace the whole engine with SI engine from the same make and model.

Many? Not many at all now. Nearly all diesels are direct injection these days.

I would agree that it makes no practical sense. Too many modifications required.

Tim

What about retooling a diesel production line to make petrol engines? ISTR a piece by a major manufacturer saying it wouldn't make much difference (in the context of the upcoming diesel restrictions).

That is a totally different question! At a guess it would not be too difficult, in that it would be a question of different parts coming in and being assembled, both engine types have similar parts, but most (read practically all) are not interchangeable between petrol and diesel. The only engine I know of for certain that used the exact same block for both fuels was older Land Rovers.

I presume you're talking of the B series.

In which case I thought there were minor changes between the two although superficially the same?

the petrol engine block of my Series 3 2.25 had the mounting for the diesel pump on the side. I don't know very much otherwise about the differences.

Thanks for intersting replies. Things always seem impossible at first. But if diesels moves out of favour, I would imagine that some interprising companies would come up with special manufactured conversion kits, not just parts that you can find. After all, the diesel blocks are stronger than the equivalent cc petrol or lpg engines, so the block should be reusable. Gearing would be high, it would not be perfect, but as an alternative to dumping a perfectly good car, also in a scenario where the diesel car market collapses.

That scenario is so unlikely it would not occur. All general production cars are just temporary transport that goes to scrap after 10 - 20 years.

I seriously doubt that a) diesels would become redundant or outlawed overnight and b) that it would *ever* make economic sense to convert an engine from diesel to petrol.

Given what's been said about the difficulty of conversion already, I'm really not sure how you've come to that conclusion.

An engine swap maybe, conversion, no.

Tim

An alternate power source completely, such as an all electric or hybrid setup would be cheaper to implement than a conversion kit, which would be so limited in likely sales that its costs would be astronomical compared to the value of the car.

Diesels will not disappear in our lifetimes, although they may / are banned in some city centres, if you need a vehicle for such use (and have a luxury diesel barge that you want to keep) then it would make sense to just get an extra vehicle just for that purpose, such as a second hand prius or rechargeable which would be far cheaper than any conversion would be.

I agree; re-tooling the line sounds relatively easy, re-using the parts much less so. Diesels have much higher compression ratio than petrol. Coping with the "geometry" is probably just tweaking the pistons and perhaps con rods. But IIRC diesels have larger big end bearings to cope with the higher stresses. Modern petrol engines get their efficiency by running at higher revs than diesels too (meaning they need lighter parts).

Agreed. And I don't see vans, buses, and lorries changing all that rapidly (city centres excepted).

There was some bollocks on the radio yesterday where someone claimed the Government should compensate diesel owners for following the earlier suggestions that diesels were "greener". I have always applied a very large scepticism filter to "green advice". (Not to say that it is always rubbish: roof and cavity wall insulation makes perfect sense).

Actually, the government can relatively easily apply incentives to change national behaviour, either via fuel or road tax. Locally, I'm happy for cities to apply congestion / pollution charges as they see fit.

Some petrol and diesels are (were) made on the same line.

Nissan CA16/18/20 SOHC/DOHC SI

Nissan CD17/20 There are significant differences between the SI SOHC and Diesel SOHC. SI is x-flow with rockers and screw tappets, Diesel has valves in-line, uses bucket tappets, inlet/exhaust ports on one side and glow plugs / injectors on the other.

Diesel CD20ETi Compression ratio, 22.2 : 1. Bore x Stroke, 84.5 x 88 mm SI CA20E Compression ratio, 8.5 : 1. Bore x Stroke, 84.5 x 88 mm.

As Turbo Diesels now have huge BMEP at 1500-1750rpm they tend to need Iron blocks. While SI turbos make more power but don't hit peak BMEP until around 3000-4000rpm so can use a light alloy block

I'm around the sort of people that put odd engines in to cars. Probably best known as hot rods or whatever. And it's a labour of love. Almost certainly not commercially viable. Unless you live in a country like Cuba where labour is cheap and new parts and cars expensive.

Until you've ever attempted this sort of job, you'd likely never guess just how many snags will crop up. Even more so with modern car electronics. Which very few home mechanics indeed understand.

Depends what you mean by 'green'. Diesels were meant to save the planet by producing less CO2. Which they do. But poison humans in towns instead. And less humans probably would help save the planet. ;-)

Even engines from the late 1980's early 1990's with comparatively simple EFI the only effective way to swap them is to get a "front cut". This gives you the engine, engine bay loom and the ECU in the passenger foot well (Nissan Skyline RB26DETT or RB25SET or Lexus V8 into 200SX). Then you have to disentangle the engine wiring from the car body wiring and make connections to power, switched power, start, tacho, speedo.

Since the advent of CANBUS the ecu now has car body functions including ABS/DSP/ESP and dash instruments, it will be virtually impossible to do a swap. Probably have to have 2 ECU's keeping the original body ECU and the new engine's ECU. But that raises the problem that both want to the instrument cluster. The alternative of locating every module the ecu controls or "talks to" and swapping that over would be very time consuming.