As illustrated by this list of car engine names by manufacturer, car manufacturers like to give the petrol and diesel variants of their engines often confusing names.

The least confusing of these naming schemes is to refer to diesel engines with d, as in BMW 525d, and petrol engines with i, as in BMW 525i. The above article shows that BMW, Citroen and Mazda all refer to the petrol variants of their engines using i.

D for diesel is self-explanatory, but what exactly does the i stand for when referring to petrol?

  • 2
    It stands for Injection as in Fuel Injection.
    – method
    Commented Apr 20, 2017 at 4:06
  • 2
    @method you should put this as an answer, before someone else does so.... and I will vote for it.
    – Solar Mike
    Commented Apr 20, 2017 at 6:21
  • Are you repairing a vehicle and are unsure whether it has a petrol or diesel engine? Can you tell us the model, maybe we can help...
    – cory
    Commented Apr 20, 2017 at 13:32
  • 3
    I'm voting to close this question as off-topic because there is no maintenance or repair question. This is a question about BMW marketing if anything.
    – DizzyFool
    Commented Apr 20, 2017 at 14:44
  • 1
    @Hashim - Don't worry too much about the vote to close. There are certain individuals on here who believe they know better than what has been accepted as on-topic questions, so continue to try and close questions such as yours, which are directly related to motor vehicles. As the only automotive Stack here, it makes sense to have these types of questions, which has been vetted on Meta in the past. No clue why certain people keep trying to close questions like this. Trying to give/gain knowledge is what this Stack is all about. These types of questions just spread more of that knowledge. Commented Apr 21, 2017 at 2:47

3 Answers 3


The i is a leftover from the 1970s, when a few companies (BMW and Mercedes, notably) started replacing the carburetor on their petrol engines with fuel injection. The marketing department saw an opportunity (for bragging about the superior fuel management on those cars) and added the i (or E, in Mercedes' case, for Einspritzung, German for injection) to the car's type number.

This worked for a few years, then the catalytic converter became mandatory and everyone switched to fuel injection. 45 years later, BMW still uses the i (as in '320i'). To make things more confusing, they are labelling their electric cars as i3 and i8.

Mercedes dug themselves a hole with their old naming scheme (which became increasingly convoluted with the introduction of the 190), so they switched to using letters (C, E, S etc.) indicating the model range. The 300 E became the E 300. So their midrange product is now called the E-class, and you can have an E-class diesel.

Similar things happened with other technologies. Turbocompressors had a T in the type name (esp. turbodiesels, TD), then came intercoolers (TDic or TDI), direct fuel injection (GDI, *SI). Most of these disappear when they become ubiquitous, some endure.

The list itself is a bit misleading. Take this entry for example, for Honda:

Petrol: i-VTEC
Diesel: i-DTEC
Hybrid: IMA Hybrid

Honda doesn't call all its petrol engines i-VTEC, just the engines that use the i-VTEC variable valve timing system. The same goes for other entries in the list: lots of them are specific technologies (or engine series), not generic petrol/diesel indicators.

  • I thought BMW had stopped using the "i" back around 2013? Commented Apr 20, 2017 at 18:44
  • Just checked the BMW site: they still use the 'i' for petrol models.
    – Hobbes
    Commented Apr 20, 2017 at 19:36
  • 1
    Just for fun, here's a good description of the whole BMW line-up. I'm sure there are other articles for the other manufacturers too. Commented Apr 20, 2017 at 19:52
  • The BMW i3 and i8 are very much electric/hybrids
    – Rory Alsop
    Commented Apr 20, 2017 at 20:49

It stands for Injection as in Fuel Injection.

  • With some minor exceptions. Commented Apr 20, 2017 at 9:46
  • @SteveMatthews: I'm struggling to come up with even one exception to i meaning fuel injection. (Honda i-VTEC comes to mind), do you have any other examples?
    – Hobbes
    Commented Apr 20, 2017 at 20:16
  • Alfa Romeo Sud Ti in which it stands for "inlet" Commented Apr 20, 2017 at 20:37
  • @Hobbes did your own answer not point out that i in TDI meant intercooler?
    – Solar Mike
    Commented Apr 20, 2017 at 21:17
  • True. I tried finding counterexamples among petrol engines.
    – Hobbes
    Commented Apr 21, 2017 at 7:26

My understanding is, the "i" stands for intercooled, particularly in turbocharged engines. A way of reducing the charge temperature to make it denser, therefore, able to react with more fuel, resulting in more power from more complete combustion

  • 1
    No. It is and always has been injection.
    – Chenmunka
    Commented Aug 7, 2023 at 9:43

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .