chingon

Just wondering why gas engines need a throttle body at all? Diesels don't and they control 'throttle' response by controlling fuel. Some direct injected gas engines don't either, so why do we need them in port injection? I read something about metering in DI injectors being better, but I somehow don't buy that. Somehow vvt comes into play here, but I don't see why if older diesels don't.

Diesels seem to be more tolerant of A/F ratios far from stoichiometry, and DI gas can get away with very lean conditions (which I guess is safe from the cooling effect of injecting fuel onto the piston).

As it stands, EFI systems rely on a TPS signal, so you obviously need it...but having a maf and map signal should be enough to 'rewrite' an algorithm.


thoughts?

Kaifd3s

Shit is about to get real.

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chingon

insightful and eloquent

pacotaco345

I'm sure there's several more reasons why your theory wouldn't work (I'm not calling you stupid by any means) but one I can think of from the top of my head are all the systems that require vacuum to operate correctly. If you have a huge gaping hole in your intake plenum 100% of the time you won't ever pull vacuum at idle, or half the other throttle range for that matter.

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chingon

jake brake or vac pump would fix that

NISR20MO

You can't control the amount of air going into the engine without one. Thus you have no control over anything, because you NEED to supply a safe amount of fuel. A safe amount of fuel to air in a gasoline engine is a pretty narrow window AFR wise, and it's now out of your control.

NISR20MO

The only way to ditch the throttle body would be to have complete and infinite control of each individual valve's lift, duration, and centerline relative to opening/closing. Which would essentially be like having a ton of tiny individual throttle bodies, doubling up as valves... Sounds expensive to me

Ilya

no throttle body = infinite red line! imagine that....

pacotaco345

LOL VVL with 2 cam profiles, idle and WOT...

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chingon

right, so I'm saying control fuel instead...like in DI diesels. You have a pretty good idea of air coming in (from MAF and MAP), so fuel alone would be what's regulated in this scenario.

There are gas engines out there w/o one, fiat's multiair and bmw's valvetronic for example. They do however have infinite valve timing and are direct injected, and I'm trying to understand why both are needed (seems to me like both injector types have precise enough metering) and diesels did w/o variable timing.

I suppose since there's no extra restriction to air at any rpm (except the valve and camshaft duration), running lean would be the biggest challenge...and perhaps compensating with fuel would decrease economy

chingon

it's called a governor

Def

The AFR range for spark ignition engines can't be anywhere near as wide as a compression ignition engine like a diesel.

The cars without a TB all use variable valve lift to meter air with the intake valves.


Won't ever happen, the fuel/combustion process is inherently different (constant volume vs. constant pressure for spark vs. compression ignition engines).

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chingon

Been a while since thermo, but I'll be damn you're right






although...you can have heat addition w/DI during the power stroke much like a diesel I believe? Which still renders my idea of port injection throttless moot anyway

edit:

on further review, isentropic compression and expansion occur regardless of throttle body being there or not and is basically all about valve opening/closing, so I don't see how that comes into play.

It seems like it's what you said and I initially suspected...AFR on diesels can go from 18 to 145 (while idling) to 1 which is running on the lean side of things
http://www.autospeed.com/cms/article.html?&A=108104
http://www.bankspower.com/techarticl...-Todays-Diesel

basically, you gotta have inlet valves sized enough for max airflow at peak power (wherever that is in the rpm range), but at idle you'd be oversized and running way lean hence the need for throttle. Talking of course w/a good old no frills cam. Not to mention when and where the fuel mixing takes place (being an air/fuel mixture in gas's intake stroke, and air only in diesel)