Why VVT is never used in diesels?

Question

There is only a mitsubushi diesel engine that uses VVT but other than that there is no diesel engine that uses VVT or VVL. Why gasoline engines do utilize them but diesels dont?

Answer 1

The "traditional" impetus for VVT - providing suitable valve timing to support the different air demands across a wide range of engine speeds (and therefore not compromising performance for either lower or higher revs) is less of a concern for diesel engines, which typically have a much smaller engine speed range so it's much easier to find a compromise valve timing that suits the operating range reasonably well.

If you consider a Honda VTEC engine such as the B16B found in the 1997-2000 Civic Type R the VTEC engages at ~ 6,100pm with the engine redlining at 8,400rpm! Diesel engines (for the most part) generally don't get anywhere near that sort of rev range with peak power for a performance diesel often coming around 3,500 - 4,000rpm and peak torque available between 1,500-2,000rpm so rather than having to accommodate a usable range of nearly 7,000rpm like the Civic it's more like 2,500rpm of range.

As Wren T's answer points out Mitsubishi have produced a VVT diesel engine for consumer cars the 41N1, the motivation here is different however - the valve timing is being controlled so as to improve efficiency and reduce NOX emissions for the Euro5 (and similar) emissions standards for diesel cars. This underlines that it's not that you can't employ VVT techniques on diesel engines, it's just that in the past there wasn't the motivation to do so.

Answer 2

I don't know for sure, but I'll give you my educated guess as to why they don't use VVT on diesel engines.

The purpose of VVT on gasoline is to provide better low end torque and then switch over (at some point in the RPM range ... in the case of Honda's VTEC, that's ~4000 rpm) so it provides more high end horsepower. In the case of diesels, they are made to produce low end torque already and never get into the high end range as I'm describing for gas engines. To introduce VVT type events into the equation on a diesel engine would not provide any benefit. Most diesel engines do not rev high enough to take advantage of something like this. Without benefit, there's no reason to use it.

Answer 3

There are engines that are diesel and use VVT. Here's an example: https://en.wikipedia.org/wiki/Mitsubishi_4N1_engine

I also came across a few scholarly abstracts that talk about applying VVT successfully to diesel engines.

Answer 4

Compression ratios for gasoline engines vary from 8:1 to 12:1 with various complications from vvt, super or turbo charging to increase power. Diesel engines have around 20:1 compression ratio and require robust mechanical designs to handle high compression. VVT may be a one-off adaptation as it's not universally adapted to other diesel engines. Diesel electronic injection is the last improvement with SCR and DEF to control exhaust pollutants. Gasoline and diesel engines operate differently with engineering tailored to extract the most thermal efficiency from both fuels. As it is, thermal efficiency remains around 25%-35% with the rest as waste heat dispersed in oil, coolant and exhaust. Diesel engine thermal efficiency is between 42%-50%, much higher than gas engines. There are pros and cons for gas and diesel engines.(edited to reflect updated higher compression ratios from replies)