Hey!

Ok, let me preface this with the fact that I have a 2000 Excursion with a 6.8l V10 (when we bought it my parents wouldn't spring for the diesel )

But, personally, I LOVE diesel engines, but don't know much about them, so I have a couple of questions for anybody who wants to take a crack at them. Any help is appreciated!

•Why can't diesels pull vacuum? In the superdutys with gas engines, all the auto-hubs, A/C, etc... are all run off of vacuum from the engine (if anybody wants to explain exactly how that works, that would be awesome too. I think its just a diaphragm in the intake though, right?) but in the diesels, all these systems have to be run off of a separate vacuum pump. I don't understand why diesels pull any less of a vacuum than gassers do, because there has to be some nexative pressure, or the air wouldn't flow into the engine. I dont get it.

•Everwhere I go, i see somebody idling a diesel truck. I see truckers all the time and busses who go and stop for lunch and just leave the truck/bus idling. I rarely if ever see this in a gasser. Why do people with diesels idle instead of shutting 'er down? I know that the higher compression puts more stress on the starter, but is this it? Somebody told me that diesels idle extremely efficiently. Is this true, and if so, why? I've asked a bunch of people this, and nobody has given me a straight answer, so I figured Id try y'all.

•Why are all the diesels I see turbocharged and not supercharged? Do diesels possess special traits that make them better to turbocharge like higher exhaust pressures, or is that just the nature of the beast that most diesels just happen to be turbos?

•How come diesels lucked out and got all the cool sound and torque and black smoke and us gassers got stuck with just an average sound, and no smoke (unless the engine's going south)

Ok guys, I know thats a lot, but I'm curious. So any bits and pieces that you can answer would be great!

Thanks!

Isaac

 

because the intake on a diesel is open. there is no vacuum in the intake to speak of, so you need a remote vacuum pump.

cause they are lazy. if i am going to be sitting for more than 3 minutes, i shut it down, and instruct all my drivers to do the same.

the older 2 stroke detroit diesels were supercharged.
4 stroke diesels have turbos.

cause real trucks don't have spark plugs

 

Really the cool sound is the compression of the engine plus the turbo. The black smoke is the unburned fuel. I try to avoid this (black smoke) on my trucks and tractors. I check it out to see why I have the smoke.

 

When I'm towing a heavy load I'll leave my diesel idle for about 10 minutes to let the turbo cool down, but that's usually more a summer time thing. I think more more guys are realizing they don't have to leave them idling. Most times I shut it down, unless I'm only stopping to grab a coffee for a couple of minutes.

 

Yeah, I agree, a lot of black smoke is not only bad for the enviornment, but also a sign of a poorly running engine. However, I think a little bit under really hard acceleration (ie passing on the highway) looks pretty cool and isnt that abnormal right?

Anyways guys, thanks all for the replies. I am stuck out of town with my grandparents, so I'm really bored, and I figured I'd post some of my nagging questions. Thanks for all the valuable info!

Isaac

 

Yes I know the feeling - our son is home for Christmas and is also bored...

 

ok, I just thought of one more question...

How do diesels control rpm and throttle? On gassers, the throttle cable from the gas pedal (or diesel pedal ) connects to the throttle body butterfly valve that indirectly effects the amount of fuel injected. But on diesels, there is no such throttlebody, so is the 'diesel pedal' only conected to the amount of fuel injected into the cylinders, eventually changing rpm as the more fuel makes a more powerful 'explosion', or is there some other form of rpm control?

Thanks again You guys are incredible source of information!

Isaac

 

Gas engines control speed by limiting the amount of air with a throttle plate, so gas engines normally run a little rich (have more fuel than the air can completely combust). so by adding a little more air, you can get a bigger bang.

Diesel engines control speed by the amount of fuel injected, so diesel engines typically run a litttle (or a lot) lean. Since there is already more than enough O2 in the cylinder, you can get a bigger bang by adding more fuel. Diesels run higher compression ratios to induce the compression combustion of the fuel, which leads to higher exhaust pressures makeing them very suitable for the addition of a turbocharger.

 

Most owner operators, in the over the road world, won't shut them down because it can get them into trouble. If something happened and they missed an appointment it can be bad. Most trucks burn around 1 gallon a hour and they won't take a chance on the truck not starting back up. Jump starting a big motor isn't that easy with a small vehicle. It can take awhile. Plus it is their house. They have things on in their sleepers that require the truck to stay running.

Marry Xmas

 

Sorry, no.

Gas engines do limit the amount of air with a throttle plate, but they do not run at all rich. They would never be able to meet the emissions laws if they ran rich, even a little bit rich.

The computer measures the airflow into the engine and injects the proper amount of fuel each time an intake valve opens.

 

Casey- it would be true of a pre-computer vehicle. With points and condenser and vacumme advance.

 

Sorry, no.

They always run a little rich (at least compared to a diesel), that's why they produce quite a bit of carbon monoxide and a diesel doesn't.

 

If they produced quite a bit of carbon monoxide they wouldn't meet the emissions laws. They do produce some carbon monoxide because a gas engine needs to run at or very near stoichiometric. That means they run very close to the ideal ai/fuel ratio. Diesels always run lean, except when there is visible smoke. And you won't see visible smoke on an emissions legal diesel anymore. And that's why diesels don't make CO, they are lean all the time.

Gas engines use closed loop control with the O2 sensors measuring the amount of O2 in the exhaust. They will hunt back and forth over very small window, between rich and lean. They don't stay on the rich side waiting for acceleration. If they always were on the rich side the O2 sensors would never read, because there is no O2 in a rich exhaust, the fuel burned it all.

Gas engines run at the ideal fuel/air ratio, diesels run lean. Neither runs rich all the time.

 

Oh. So why do we still need a catalytic converter on gas engines?

 

Gas engines run at stoichometric (14.7 A/F ratio unless they are running WOT or on the warm up circuit on a cold start which then they will run allitle rich)