Why no boost til loaded?

[SWendell]

Wondering... ... ...

Why is there no boost produced with increasing RPMs while the truck is in park or neutral? As soon as a load is placed on the motor, the boost spools on up. It must be a simple explanation, but I cant work it out.

Thanks

Scott

 

[RustyJC]

It takes exhaust gas horsepower (EGHP) to drive the turbocharger. EGHP (manifested as exhaust pressure - aka "drive pressure" and EGT) is a function of the amount of fuel being injected into the engine. This is next to nothing at idle or when revving the engine at no-load. As more fuel is injected with increasing loads, more exhaust drive energy is available to the turbocharger, and more boost is produced.



Rusty

 

[SRadke]

It takes fuel to spool!



-Scott

 

[Gary - K7GLD]

ACTUALLY, the "no boost 'til loaded" is a big plus for a turbocharged engine, since low/no boost allows the engine to operate at a much lower stress level than it does when heavily loaded - an UNstressed engine can operate more economically and with a longer lifespan than one that runs around heavily loaded a high percentage of the time...

 

[SWendell]

So... if I put the pedal on the floor while parked in the driveway, there will be less fuel injected vs flooring it pulling out of the driveway? How does the pump "know" if there is a load or not? Seems a purely mechanical linkage to me



I got to thinking about this after I got my tires balanced... . the tech pulled my truck out of the bay and I saw him revving it up and checking out my gauges... . he probably thought the gauge didnt work!

 

[Gary - K7GLD]

Stop and think about it, WHICH requires more POWER from the engine, reving it at a standstill in yer driveway, or accellerating a 7000 lb truck from a dead stop?



I'll give ya 3 guesses...



And the engine "knows" the difference between the above 2 situations because of various sensing controls and the basic fact that more fuel under load = more expanding heat to drive the turbo harder...

 

[rbattelle]

I believe it will put the same fuel into the engine when you floor it unloaded as it would floored and loaded. However, the problem is, you have to let off the accelerator too quickly (prevent from exploding the engine) to allow the turbo to spool up and make boost.



Someone smarter will correct me if I'm wrong, but I believe fuel delivery is in direct proportion to accelerator pedal position rather than a function of pedal position and load. Especially on a non-electronically controlled engine.

 

[SWendell]

Originally posted by rbattelle

I believe it will put the same fuel into the engine when you floor it unloaded as it would floored and loaded. However, the problem is, you have to let off the accelerator too quickly (prevent from exploding the engine) to allow the turbo to spool up and make boost.



Someone smarter will correct me if I'm wrong, but I believe fuel delivery is in direct proportion to accelerator pedal position rather than a function of pedal position and load. Especially on a non-electronically controlled engine.

Thats what i was thinking.



How about this theory... . with a loaded engine, the boost is kinda backed up from the load that is transmitted through the wheels to the pistons... thus creating a sort of back pressure versus an unloaded motor... the air flow/boost is not impeded, as the motor freely revs. Boost is created, but lacking any resistance, it escapes easily out the 4" pipe.

Any logic there

 

[Gary - K7GLD]

I believe fuel delivery is in direct proportion to accelerator pedal position



UMMmmm - not exactly - it is ALSO determined by a function of intake manifold pressure and engine RPM compared to accellerator position - that's why on the older VE pumps with the vacuum-controlled fueling pin/cam, you would rotate that diaphram and attached pin to increase fueling under varying manifold pressures - which varied according to engine load and RPM. The pedal was the primary "adjustment", but OTHER engine parameters came into play to increase fueling as well. And as fueling increases against load, the expansion of exhaust heat generated because the engine is doing more work and using more fuel. causes the turbo to spin faster generating more boost.



Conversely, with the truck at rest, reving the motor against little or no resistance is causing little/no "work". and fueling remains comparatively low, and less expanding heated exhaust gasses = little boost. But as was said above, a QUICK stab on the accellerator WILL drive boost upwards momentarily as the engine is forced to work harder to accellerate to a higher RPM - but that accelleration in RPM happens so quickly, not nearly as much boost is generated.

 

[nickleinonen]

if you were to look at the rack settings for an engine while at idle, and at top idle [on the governor] there is very little difference between the fuel settings. if you are sitting at low idle with a hot engine, and you pin the throttle down all the way [wot] in neutral, you will see a little boost build but as soon as it hits top idle, the boost will drop right away as the governor backs the fuel off to maintain the top idle [say 3200rpm]

 

[SRadke]

Pedal position is proportional to rpm, not fuel delivery. These computer controlled jobs have a little diffrent govenor but that's how the old tractors are. Give them whatever throttle (rpm) you want and it add's all the fuel the rack will allow it to as the load increases trying to maintain that rpm.



-Scott

 

[HoleshotHolset]

FWIW: VGT/VNT will help give you boost with no load...



My VW TDI would make 20psi with the transmission in neutral and ~4500rpm.



Boost is a function of RPM, drive pressure and temperature delta -which is affected quite a bit by the shape, size, geometry, and type (VNT/VGT/traditional) of turbine housing in use.



Matt

 

[jwgary]

First, pedal position does not = RPM like on a tractor. If you plant your foot on the transmission tunnel so it does not move, you will lose speed as you climb a hill.



Second, the amount of fuel at full throttle will be limited by the governor once you reach governed speed. If you are in neutral with no load, it takes almost nothing to reach governed speed and so little fuel is being used. If you are on the floor with a big load going up hill, you will probably never reach governed speed and therefore will get all the fuel it can deliver.



Boost pressure is directly related to cylinder pressure. The greater the pressure of the exhaust, the more pressure there is to spin the turbo and therefore create more boost. Under no load, the cylinder pressures are low.



I hope that clears up some things.

 

[SWendell]

Things are making more sense now...

I didn't realize the p7100s were that sophisticated. Well... I suppose they are not near as complex as the computer controlled rigs, but ... not quite as simple as I have been assuming.

-----Thanks for all the info

 

[klenger]

Smart guys who thunk all this stuff up.

 

[Greenleaf]

Maybe I missed this.....

... but did someone explain that if you floor-board the accelerator w/transmission in neutral the engine will not blow up?



FWIW, Mine will stop at 3,400 rpm's w/no or little boost pressure.



Scott

 

[Gary - K7GLD]

... but did someone explain that if you floor-board the accelerator w/transmission in neutral the engine will not blow up?



It's called G-O-V-E-R-N-O-R!



Not many would feel comfortable doing it, but you CAN hold the pedal to the metal with NO engine damage because the govenor regulates max engine RPM...



About the only way to overspeed the engine and cause damage is to come down a grade in lower gears and actually FORCE the RPM above redline...

 

[Greenleaf]

That's right Gary. We lost a Cummins N14 that way when I used to work in the oil fields. Guy thought it was one of them Detroit Screamers and scattered 855 cubic inches of Cummins iron



You gotta luv them governors. Is it the same as "Arnold"



Scott

 

[rbattelle]

I knew someone (or several people) smarter than me would correct me. Thanks, guys!