Diesel/Petrol engine sizes in cars

I’ve notices that for the most part, below 2 litres in capacity, cars petrol engines seem to come in even sizes; 1.2, 1.4, 1.6 etc and diesels in odd numbers; 1.5, 1.7, 1.9 etc

Now, I know that if the badge on the back of my car denotes an engine size (in my case) of 1.4, it means that the engine size is close to that (again in my case, its 1397 or something similar.) And I know that there are other diesels such as the new VAG 2.0 litre diesel.

But is there any reason why roughly even numbers should be preferable for petrols and odd for diesels? 1.9 in particular seems popular for diesels, any reason for that?

Coincidence, and maybe subconscious selective noticing. I’m quite confident that engineers are not selecting such numbers, and that design goals do not dictate them. Then there’s the matter of which number is even - in this case you’re talking about the 1/10 of a liter. What if the engine sizes were given to the 1/100 of a liter? What if they were given in cubic inches? It’s all rather arbitrary.

The popularity of certain engine sizes may be different here in the U.S., but among the gasoline (petrol) engine sizes in a booklet at hand are 1.3, 1.5, 1.7, 1.9, 2.3, 2.5, 2.7, 3.3, & 3.5. There are plenty of 1.6, 1.8, 2.0, 2.2, etc. engines as well, but for this parameter of the 1/10 liter digit being odd or even, I don’t think you’ll find any explanation more accurate or satisfying than “that just happens to be what it ended up as.”

They are rounding off to easy numbers for marketing. My 750 CC motor cycle was really 748.something. The nissan 350 Z has a 3.498 liter engine but it is widely marketed as a 3.5 liter engine.

Accurate but not satisfying :wink:

On another note, perhaps engineering related, is 0.5 of a litre per cylinder the maximum desired capacity? Just a thought, engines bigger than 2 litres seem to be 6 cylinders more often.

Well, the Harley-Davidson “88” engine has two cyclinders totalling 1450 cm[sup]3[/sup]. And I’d draw your attention to this gasoline truck engine: the 8.2 liter V8 (8194 cm[sup]3[/sup]/500 in³) in the 1970 Cadillac Eldorado 500.

Diesel engines, of course, can be larger – ever seen the pistons on an oil tanker?

Automakers used to simply make a diesel motor out of a gas engine by increasing the stroke (they also used flat topped pistons and flat heads to minimize combustion chamber volume) to increase compression ratio high enough to suppore compression ignition.

A deeper draw on the intake and higher compression ratio may add up to a tenth of a liter over a gas counterpart.

The main reason is taxation. I couldn’t find any good information, but this wikipedia article will give you some insight: http://en.wikipedia.org/wiki/Engine_displacement#Governmental_regulations

There is no technical reason for a gas motor to be even sized, and a diesel to be odd.
My company builds a line of engines that share most components. Over the years they have built 4, 5 and 6 cylinder versions. The displacements have ranged from 1.9l, 2.0l, 2.3l, 2.4l, 2.5l 2.8l, 2.9l, 3.0l. (all imported into the US) For other markets, there were also some other sizes that I am not familar with. All of these were gas motors. I should note that the rules for rounding changed, and the 3.0l and 2.9l are exactly the same motor. The true displacement was 2938 (IIRC) then the rules changed, and it had to be marked a 2.9l for legal reasons here in the states. :slight_smile:
There were also diesel versions of these engines (not the exact same motor, but a design that shares much of the same layout / attachment points common accessories. displacement on these engines are either 2.4l or 2.5l.
Enola Straight The only people that made a gas engine into a diesel this way that I can recall is GM. That engine went over like a turd in a punch bowl. :eek: :smack: On those engine I listed above the 2.3l and 2.5l were both the exact same stroke, the 2.3l had a 2mm smaller bore for extra strength as it was the turbo version.
Dog80 the Wiki article missed the Italian market. I have been told (no cite sorry) that engines over 2.0l in Italy are taxed out the ass. So my company makes our various model engines with a 2.0l displacement for Italy.

When VW introduced the Rabbit diesel, didn’t they claim such-and-such percent interchangeability between the diesel motor and gas versions?

I’ve owned cars with both a 2.2l & 2.6l 4-cylinder engines. Both of 'em turbo-charged, too.