Why you can slap a turbo on a diesel easy and not so on a gas engine

[FrankenDiesel]

Hello,

I have been interested on fi for the past few years and have always been reading on tuning afr. One thing i am a bit cloudy on, but somewhat understand is why diesels take on a turbo so easily?

Diesels are easily double the cr of a na gas engine, so one would think that there would be detonation or pre ignition. If you put a turbo on a gas engine its a ticking timebomb without a tune, where as a diesel is more forgiving.

Before really owning a diesel i thought the answer was simple: a gasser has more of an explosion due the the volitality of gas while a diesel would be more or a burn, which is completely opposite (in the sense that diesels boom and gassers burn).

I have come to the conclusion that with a high cr, these diesels are designed for extreme preasures whethor it be construction of the engine or make of the internals. (Which i assume are forged agross the board ). Lastly and arguably most important is fuel in a diesel only enters the combustion chamber when it should be fired, where a gas engine has fuel that. An theoretically preignite during intake and compression stroke, yielding a big explosion.

Anyways i know i have thought about it, so others must have had the question . Did i hit the nail on the head or am i missing/incorrect?

[CRSMP5]

Sequential injection is biggest reason.... Gas sprays into all holes on gasser vs diesel

[vanbcguy]

Preignition is not an issue on diesels as the fuel isn't put in to the cylinder until it is needed.  As soon as fuel is injected it starts to burn - that's the other part of it.  With a gasoline engine the entire fuel load is in there from the beginning meaning the whole thing can go at once, whereas with a diesel the injection event continues for several degrees of crank rotation.

The other BIG part is that diesels come at the AFR from the opposite side of stoichiometric from gassers.  A gasser usually runs closer to stoichiometric - they generally can't go leaner than about 15 or 16:1 without extremely high temperatures.  In most gasser turbo engines the designers have to richen the mixture to bring the temperatures down - it is not uncommon to see 12:1 under higher power.

Diesels on the other hand run extremely lean - idle is often around 100:1 while absolute full power is more like 18:1.  The more air you put in a diesel the cleaner it burns and the cooler the in cylinder temperature, completely the opposite of gassers.  That's the big part of why they like turbos so much.  Feeding more air in will reduce the chances of melting pistons and things (within reason) but won't cause preignition, since there's no fuel to ignite till it is supposed to be burning.

The high CR is mainly to do with getting the engine started more than anything.  The compression ratio has to be high enough that the air being compressed in the engine gets hot enough to ignite the diesel mist on its own even in cold weather, something you can't do with lower compression ratios.  Lots of race-only diesel engines run much lower compression ratios but the usually can't start without ether / a blowtorch / etc. fed in to the intake.  Glow plugs provide a hot spot to help but they aren't enough to light things off on their own without sufficient compression heat.

[jhax]

Very interesting stuff. My friend and I were discussing the other week why the VNT turbos werent used on many gasser engines as opposed to diesel engines. I know there were some gassers that used VNTs but it never really caught on. I assume this also has to do with the stociometric pressure differences. Are you saying that if a gasser gets more air it runs hotter due to the very short window of ignition? As opposed to a diesel which runs cooler as more air is forced in? We were really scratching our heads with this one.

[air-cooled or diesel]

its not the air/ its the combustion process; completely different from gas to diesel. the high compression in a diesel is for sustained complete combustion. in a gas more air means you need more fuel/more air alone is lean; it ll run hot and hotter.
a turbo to a diesel is like bread and butter; in a gas you have a lot to compensate for hi-air flow; in a diesel you just need to route the air-flow. 

[theman53]

Sequential injection is biggest reason.... Gas sprays into all holes on gasser vs diesel

Yep, if you had a direct injected gasser, no throttle plate, it would probably work just as the diesels do...VW that is as we have all forged internals from the factory.

[745 turbogreasel]

A stock 5.3 Chevy motor with a Chinese  turbo and no intercooling will give you  500+ HP pretty reliably.
I don't see how that's much harder than  +T'ing a VW.

[theman53]

A stock 5.3 Chevy motor with a Chinese  turbo and no intercooling will give you  500+ HP pretty reliably.
I don't see how that's much harder than  +T'ing a VW.

reliably for chevy standards I would agree 100% with you.