Engine braking Gas Vs Diesel

brulaz wrote:

kyle86 wrote:

Me Again wrote:
I wonder how the new direct injection gas engines are doing that have no butterfly valve? Audi, BMW and Mini Cooper all have engines in production with DI.

I was under the impression that DI just places the fuel injector in the combustion chamber instead of the intake runner. They should still have a throttle body to let air into the engine. I could be wrong.

Apparently some eliminate the air throttle plate according to Wikipedi anyway:

Wikipedia on GDI wrote:
In addition some engines operate on full air intake. That is, there is no air throttle plate eliminating air throttling losses in some GDI engines, when compared to a conventional fuel-injected or carbureted engine, which greatly improves efficiency, and reduces piston 'pumping losses'. Engine speed is controlled by the engine control unit/engine management system (EMS), which regulates fuel injection function and ignition timing, instead of having a throttle plate that restricts the incoming air supply.

I'm seeing aftermarket replacements for the 3.5L Ecoboost throttle body, complete with butterfly valve, so I guess that engine does have an air throttle plate. http://www.stage3motorsports.com/1822-1-2011-2013-F150-37L-BBK-73MM-Throttle-Body.html, so that should help with engine braking.

I'm suspicious of the wiki article claim. Gas engines need to operate over a vary narrow air/fuel ratio. So there must be some way to control the air volume entering the cylinder. Throttle plate or something. It may be located downstream of where one might expect it, but still there must be a way to control air volume in the form of some kind of variable restriction on the airflow.

Diesels will operate over an extremely wide air fuel ratio, so the throttle plate is not needed to control the amount of incoming air.

Hence less pumping losses but also less restriction under no throttle operation and hence less braking.