Why do diesels need a vacuum pump? - PeachParts Mercedes-Benz Forum

chasinthesun · #1 12-20-2006, 12:36 PM

Does the engine not make enough vacuum?Gas cars usually tie into the intake for the use of vacuum needs ,Why does the diesel engine differ?

janko · #2 12-20-2006, 12:38 PM

no throttle. ergo no vacuum.

rrgrassi · #3 12-20-2006, 12:43 PM

You would have to hook up before the turbo, or onto the air intake for vacuum, but the air intake.

Probably on a non turboed car you could tap into the intake like a gasser.

Now I'm curious to see if I could make a vacuum device work like a siphon type like on a garden hose sprayer.

Hmmm....

Samuel M. Ross · #4 12-21-2006, 02:47 AM

Quote:

Originally Posted by janko

no throttle. ergo no vacuum.

There is some vacuum in the intake manifold or the air would get sucked into the engine for combustion on the engines without turbos... but you are are entirely correct that the lack of a throttle valve [as ther is in gas engines]... the lack of such a valve means there is much less vacuum [almost none at idel ]. IF you were to monitor the vacuum in the intake manifold of a naturally asperated diesel, you would NOT see the very distinct fluctuations as there are in a gas engine.

Possible Sidebar Reason - The 1930s/1940s generation of American automatic transmissions that MBZ copied in the 1930(s) when they first started building "slush trannies", well these were for gas engines and they were designed to shift based upon that same dramatically fluctuating intake manifold vacuum which is also a very good way to both measure the load on the engine and the "go" requests of the driver which needs to be communicated to the tranny.

Thus the MBZ's engineer's ingenius design for a vacuum control system on their diesels which evolved over the years up through about 1985.
Another Reason - This also allowed them to use basically the same trannies on GAS as well as Diesel engines. Much of the changes in the latter 1980(s) resulted from the engineers trying to appease the California State environmental "wackoes". If you don't believe me, take a look at the diagram for the 1985 California diesels at: http://www.peterschmid.com/vacuum/1977_1985/617_95/1985_cal.jpg.
Thanks goodness they never made a "Federal" version of this system... at least I don't think they did?... in the SDL(s)???
Regards,

MS Fowler · #5 12-21-2006, 01:01 PM

Quote:

Originally Posted by Samuel M. Ross;[B

[U]
Thus the MBZ's engineer's ingenius design for a vacuum control system on their diesels which evolved over the years up through about 1985.
Another Reason - This also allowed them to use basically the same trannies on GAS as well as Diesel engines.Regards,

As an interesting ( at least to me) note....
Ford did exactly the same thing. Their automatics--C6 and others used engine vacuum to signal the tranny when to shift.
When they installed a C6 tranny in a diesel pickup, they needed an "Transmission vacuum modulator" to convert the vacuum from the pump to a proper signal for the tranny. Its basically a variable bleed that reduces the vacuum to the tranny when the throttle is suddenly opened. Works well when it works. If it breaks, all shifts are like at WOT, meaning high in the engine's rev range and quite firm.
Not many Ford-Mercedes similarity, but this is one.

vstech · #6 12-21-2006, 01:10 PM

ditto GM with their infamous 5.7L motors VP was driven off the cam, the pump looks like an HEI distributer without all the wires....
John

rwthomas1 · #7 12-21-2006, 05:44 PM

Why do diesels have vacuum pumps? Well, not all diesels do have vacuum pumps but the ones that do generally have the pumps to provide vacuum to operate a myriad of control functions and accessories. Vacuum would seem to have been considered more reliable than electronics, etc. in engineering some vehicles from the not to distant past. MB liked to use vacuum for brakes, transmission control, emissions control, locks, ECC functions and shutting the engine off. Other manufacturers have used vacuum for the above stuff plus wipers, headlight doors, etc. RT

bullwinkle · #8 12-21-2006, 07:55 PM

Quote:

Originally Posted by vstech

ditto GM with their infamous 5.7L motors VP was driven off the cam, the pump looks like an HEI distributer without all the wires....
John

Don't forget the 6.2/6.5s, they're ran the same way-the early ones like mine just had a diaphragm sticking up for generating vacuum, then they went to a belt driven one on the later ones and used the "distributor" for an engine speed sender.

TheDon · #9 12-20-2006, 12:39 PM

plus a mechanical vacuum pump lasts longer than an electric version... reason for vacuum actuated.. everything on the W123... just look at the W116's servo control AC.. the servos fetch big money if OEM replacement parts still in the box

SD Blue · #10 12-20-2006, 12:47 PM

Gas engines have to generate a vacuum in order to overcome the throttle of the intake. This determines their air/ fuel mixture.

Diesels generate little to no vacuum in their intake, if fact, most of the time there is pressure. They use as much air as you can pump into them. That is what makes them a more efficient design.

vstech · #11 12-20-2006, 01:17 PM

Yeah, due to the restriction in the airstream in a gasser, you get vacumme unless you are flooring the pedal. However, high performance motors, like a race car, the cam profile is too large, and the motor makes poor vacumme. also in a gasser, vacume aids in the evaporation of the fuel spray, to get as much fuel atomized for a more efficent burn. a Diesel has it's fuel directly injected into the cylinder at huge pressures, into highly compressed air. this ignites the fuel as it is sprayed in, no mixture or evaporation needed.
basically, unless there is a butterfly flap to restrict air as it is pulled by the pistons, there will be no vacumme to speak of. the intake is open completely to atmosphere.
John

Shorebilly · #12 12-20-2006, 03:23 PM

Quote:

Originally Posted by tangofox007

Diesels don't have throttles to restrict air flow into the intake manifold. Therefore, the is not much vacuum in said manifold. Even less in a turbocharged diesel.

How about a pressure in a Turbocharged Diesel!!!

My first car had vacuum operated windshield wipers, as does my ol' power wagon.....I wonder how many of y'all have ever had to deal with gettin' off the gas to increase wiper speed.......

SB

vstech · #13 12-20-2006, 11:08 PM

Quote:

Originally Posted by Shorebilly

How about a pressure in a Turbocharged Diesel!!!

My first car had vacuum operated windshield wipers, as does my ol' power wagon.....I wonder how many of y'all have ever had to deal with gettin' off the gas to increase wiper speed.......

SB

I used to drive my old boss's 57 fairlane, the old doublebarrel shotgun tail light v8 312... vacuum door locks and wiper motor, something else was vacuum too... don't remember though. that car had rear pump in the auto tranny too. my boss loved to roll it down the hill to speed start it. he liked to save the load on the starter... riot.
John

probear · #14 12-20-2006, 03:31 PM

Dunno about the vacuum wipers but my 69 Lincoln had hydraulic wipers that ran off of the power steering pump. You did not want the wiper blades to hit your hand. They did have some real power..

97E300D · #15 12-20-2006, 04:00 PM

Our cars breath in a sort of reverse aquarium

It's funny how people think of vacuum or "suction"... The pistons don't really pull air in through the intake. It's the weight of the atmosphere *14.7* lbs\sq inch that pushes the air in to fill the non-air bubble or low pressure area. A Diesel will produce a small vacuum and it will build as RPMs increase, but as stated it is not enough to power things.

I always like to think of the benefits of a supercharger or turbo in this way. As the cylinders request more and more air in the same give time the atmosphere can't push it in through the intake plumbing fast enough. By the time the intake valve closes more air is depleted then can rush in. sort of like pushing allot of water through a small pipe or high amps through a thin cable.

Turbos can help to over come this... even if the pressure never exceeds the relative atmosphere, power builds as the cylinders fire twice as fast in the same time, yet can't get as much air as it could without the forced induction of air. Remember the equation work = weight moved over time.