Vacume and diesel engines

[soulasphil]

diesels are not as different from gas engines as you imply. My MF35 Massey Ferguson has a butterfly in the intake manifold to adjust engine speed and the depression caused by the downstroke of the pistons is enough to work a regulator commanding the injection pump to maintain tractor speed when more or less effort is required.
Phil

 

[Hooked_on_HP]

If AP is what is filling the cyl with air and there is no vacume why do we have vacume indicators that tell us when the air filter is getting pluged. Also, I will bet if you take the intake hose off you air filter and put your hand over it, it will try to suck it in. Is it vacume doing that or AP.I will however go along with not having usefull (high) vacume.
Bill

 

[RalphVa]

No vacuum on the suction of a diesel engine because its air flow is unrestricted. There is air flow. This is how it flows through the air filters into the cylinders.

My Benz has a separate vacuum pump for auxilliaries that need vacuum.

Ralph

 

[firedog]

This is one of those discussions that can go on forever.
(Why I said yes & no) Vacuum is a relative term. Used mostly where gauge pressure is considered (where atmospheric pressure is considered "0" psi) thus anything less is a vacuum. In the case of the cylinder, as it moves downward it creates a less than "0" pressure and is filled by the higher "0" atmospheric pressure.

When atmospheric pressure is typically used in "pressure speak" then your more or less looking at pressures in absolute where "0" pressure is -14.7 psig.
Thus the seperate terms psig and psia. Almost all equations/gas laws use psia not psig.
So, Jim is correct in his definition that atmospheric pressure pushes into the less that atmospheric cylinder. (but then, thats where the term vacuum comes in.)
In the truest sense then, the is no such thing as vacuum, only less pressure.

 

[Richard]

Maybe we can discuss an old theory about a plane and conveyor belt...

 

[Soundguy]

If AP is what is filling the cyl with air and there is no vacume why do we have vacume indicators that tell us when the air filter is getting pluged. Also, I will bet if you take the intake hose off you air filter and put your hand over it, it will try to suck it in. Is it vacume doing that or AP.I will however go along with not having usefull (high) vacume.
Bill

Take a minute and think about what you just said.

If the filter is good, there is no restriction.. as the filter gets clogged, then there is restriction to air flow.. thus the indicator shows a vacume situation.. and that the engine is starving for air...

You are talking about the exception.. not the rule.

soundguy

 

[Iplayfarmer]

If AP is what is filling the cyl with air and there is no vacume why do we have vacume indicators that tell us when the air filter is getting pluged. Also, I will bet if you take the intake hose off you air filter and put your hand over it, it will try to suck it in. Is it vacume doing that or AP.I will however go along with not having usefull (high) vacume.
Bill

It's symmantics to an extent. Think of it like light and dark. Dark is nothing more than the absence of light. Vacuum is nothing more than the absence of pressure. It's the difference between the pressures that does all the work. Without the atmospheric pressure, the vacuum will not do anything for you.

It may seem like it's purely a matter of word choice, but understanding the influence atmospheric pressure has on aspiration is important in certain situations. This is why the old snowmobilers carried extra carb jets, etc. with them when they were high marking. As they get higher in the mountains, the air thins and the pressure around them is lower. The engine is still creating the same absolute vacuum that it ever did, but the vacuum relative to the pressure around them changes. They have to change the carb jets to keep the fuel mix right because of the change in Atmospheric Pressure.

Savy?

Turbo is another aspect of the same issue. A turbo charger gives a "boost" to the atmospheric pressure allowing for greater difference in pressures. The greater difference does more work, and more air can be pushed into the cylinder. More air = More oxygen = more combustion.

 

[milkman]

I borrowed this


starting to seem fitting here too.

 

[Soundguy]

A turbo charger gives a "boost" to the atmospheric pressure allowing for greater difference in pressures. The greater difference does more work, and more air can be pushed into the cylinder. More air = More oxygen = more combustion.

More air lets you inject more fuel. Also helps correct for HP loss of NA engines at high altitudes..

soundguy

 

[Redneck in training]

Well guys, stop talking about vacuum and talk instead of throttling loos. Diesel has low throtling loss due to suction resitance being the filter, suction duct and sutcion manifold.
Gasoline engine has all of the above but it also has a throttle(s) causing much larger pressure drop and thus throttling loss.