First off, if there is a great book that might answer some of the following questions, feel free to suggest them... John.

1) How do pumping losses relate to engine performance?

2) To generalize, will an engine with zero or negative overlap take longer to rev through it's power band compared to a similar setup with a cam with say 3-5 degrees of overlap at .050?

3) Some of the more exotic cars like the Bentley Flying Spur have pcms that limit the amount of torque throughout the powerband.

"Under the influence 10.1 pounds of boost pressure, this 6.0 liter engine develops 551 horsepower at 6100 rpm and 479 pound feet of torque... The engine-management computer regulates this boost pressure to keap peak torque unchanged all the way to 5100 rpm, probably to avoid stripping the teeth of fthe gears in the six-speed ZF transmission."
-Car & Driver, Februrary 2006, page 100

So how does the tune change to regulate the torque output?

4) A two-piece driveshaft is used to keep the price of cars down, but why would the price be so much cheaper that it makes sense to do it that way?

5) Are dry sumps used to keep oil pressure due to how fast the engine is spinning, how high it's spinning, how long it's being run, or the side-to-side sloshing of oil in a car running a road course? I'm guessing the answer is the last two.

 

Ive always wondered about #5 myself... and I always thought that the 2-pice driveshaft was to take away vibrations or somethin like that, not for price.

 

#1. Pumping losses are "negative work". It takes energy to compress the air in the cylinder. For example, leave the spark plugs in and try to turn a flywheel. you can feel how much force you have to exert to turn the motor. Then pull out the spark plugs, (no compression) and it is fairly easy to turn the motor. Of course, for real engine development these are measured using motoring dynos that spin the engine and the parameters are measured.

#2 Generally the speed at which an engine rev's mainly due to the rotational inertia of the pistons, con-rods, crankshaft, flywheel. Look at the Porsche Carrera GT, a VERY low inerta engine, it revs nearly instantly, but the downside is you have to be awesome at driving stick to not kill the engine.. it would be hard to even feel when the engine died when you drop the clutch. We cant forget the influence of power in the equation, an engine with more power will rev faster with the same rotational inertia. The cam profile will be chosen to produce the most power and maximum cylinder filling and by doing that, your engine will rev faster.

#3 Flat torque curves are the epitomy of luxury and ease to drive. If an engine maintains a consistant boost pressure and the VE of the engine is nearly even throughout the powerband you will have a FLAT torque curve. There are other minor variences that the computer can control to maintian a flat torque curve such as ignition timing and valve timing but the major player is the boost pressure and VE profile. Most boosted engines do not maintain the same boost level throughout the entire powerband, usually sacrificing the flat curve for higher boost #'s in the upper rev range and thus more power. (keep in mind the boost devices have thier own efficiencies and such that can be monitored and control to maintain perfect boost #'s) and in the case of the flying spur it may be a case of saving the driveline components life expenctancy.. but thats a side point...

#4 Two piece as in 2 U-joints and a splined sliding connection of the 2 pieces or 4 Ujoints with a bearing connection in the middle? Both have different reasons...

#5 Dry sumps, they are pumps that take the oil out of the oil pan from mulitple locations and put it in a designated canister. This pump can keep working even if there is lots of air in the lines and going through the pump. It put the oil in a canister where the air can be separated from the oil and can feed the internal oil pump pure oil. A wet sump takes oil directly from the oil pan and only from one location, if the oil is not in that location (because your turning, stopping, accelerating etc..) then the internal pump will be sucking in air and feeding air to your bearings and internal oil passages which is NOT GOOD! So the dry sump has lines in different areas within the pan and will allow the oil to be picked up in all conditions (accel, decel, cornering) and during those conditions some lines will be sucking air with the others oil... then the catch can will allow the oil to be separated from all that air. And yes, the oil pumps are positive displacement pumps which means the oil flow is directly porportional to engine RPM so during that, it is essential that no air enter into the bearings and passages. The other benefit to dry sump is that no PVC is required since it can pump any blowby out and actually creates a vaccume in the crank case. This vaccume is beneficial because the pistons move downward because of the high pressure in the combustion chamber adn the low pressure in the crank case... so if the pressure is decreased in the crankcase, a higer "differential" pressure is seen from the top of the piston to the bottom.

 

The dry sump also decreases windage losses since there isn't any oil splashing/sloshing around in the pan.

 

VAG (Volkswagen-Audi), the Bentley's parent company, has used management of the turbo boost for quite a few years to build flat torque curves. The 1.8L 180 hp turbo (in my wife's Audi TT) has the flat-top from ~1900-5000. Breaking gears isn't a problem here , but it does give very smooth linear acceleration which makes the engine feel like it is a larger displacement mill, not a peaky ricer. You don't need to row it around with the gear lever, and it's not very sensitive to the gear you select at part throttle driving.

Control of boost, fuel and spark can pretty much shape the torque curve. GM tends to do it on NA engines with intake/head/valve motion design. Look at the number of GM engines with about 90% of max torque from about converter stall to max shift point. Even the LS7 exhibits this type of torque curve.


Length/diameter ratio (L/D) becomes a "whipping" problem with driveshafts. If L/D gets to ~15:1, any little bend or unbalance in the shaft can cause big problems at high rpm. Consider a 4-1 downshift in a C5 Vette when a 4-3 was desired. Without a little rubber bumper around the middle of the shaft, whipping can cause it to rub the torque tube. That driveshaft ("propeller shaft" in OEM-speak) is Metal Matrix Composite (MMC), which isn't cheap., but it still whips.

With a .50 OD on a Camaro, the driveshaft is spinning at 2x engine speed in 6th. Throw some 4.56 gears in your Camaro and then cruise it at high speed in 6th and you may "feel" the problem.

2-piece driveshafts with a CV joint in the middle also help packaging with lower floorpan humps. Two small diameter steel shafts are cheaper to build, have less inertia and are safer as well as probably stronger than one big one. Kind of a reverse "size matters" thing.

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This is a great book to read PSJ, I think it will shed a lot of light on your questions. http://www.alibris.com/search/detail...ches=4&qsort=r

This is a very advanced book, so be prepared to be stumped!!!

 

A "pumping Loss" is simply the inefficient filling of the cylinder. Pumping losses are always present in an engine. The only way there would be zero pumping losses would be if there was no cylinder head on the engine and the cylinder would be filled each and every cycle.
Pumping losses are caused by: Valves, Ports, Throttle Bodies, air filters, and restrictive exhaust systems. Basically, any work that is performed to by the piston to fill the cylinder can be considered a pumping loss.

This is why when you run an engine on the dyno you notice the manifold pressure dropping at the top of the dyno run. This tells you that the throttle body or the air filter is restricting the airflow into the engine.

But, just because the manifold pressure doesn't drop doesn't mean it is working right. It could be that the exhaust system is to restrictive and it can't get rid of the spent gases.

So, I guess I will wrap up now, but just remember, that if the piston has to use any of its motion on moving air, it is loosing power put to the crankshaft which looses power to the wheels.

 

It actually doesnt matter what gear your in, its only related to the Tire diameter, differential ratio, and MPH. Transmission has nothing to do with how fast the driveshaft is spinning at a given mph.


Now this phenomena is called "critical speed" in engineer speak. Basically its the lowest speed a shaft will excite itself into resonance. It relates to shaft diameter (inside and out), length between supports, material modulus of elasticity and density, and bearing stiffnesses.

 

Also, at part throttle your pumping losses are greater due to very low manifold pressure (bad for highway MPG), which is another reason diesel engines are more efficient, (no throttle blades for controlling manifold pressure)

 

You are correct. Camaro wasn't a good example. If you stick enough gear in a Camaro you could reach the critical speed of the propshaft at a moderate engine speed.

FWIW, in the C5 example, the prop shaft is between the engine and the trans input. 5>2 or 4>1 downshift does get "exciting".

 

Thanks for the detailed responses guys.

 

I thought the whole reason for 2 piece d/s was for front crash impact issues.The drive shaft has to be like this so it doesn't prevent the engineand trans from "Submarining"under the body/floor in a heavy front impact situation.
The shaft is designed to concertina under this type if impact.
Any comments?

 

Another consideration for the DS, is critical speed, as I discovered to my disgust when I had a 1 piece made for my car.

With a shaft length of approx 55", critical speed was a pathetic 5800-6000rpm or thereabouts with a 3" Chromoly shaft. I barely had room for that, so larger diameter shafts were not an option.
Even with 3.27 gears, this restricted me to about 120mph before a horrendous vibration entered the car. It was only after this vibration I had to research myself what the problem could be.
When anyone orders a custom shaft, the critical speed issue should be made very apparent to the customer beforehand. They shouldnt have to find out for themselves once its installed.

Why this condition was never mentioned to me when I ordered the shaft is beyond me.

If you have a car, that requires a long DS, then the only option IMO is a 2 piece. My current shaft is a 3" steel 2 piece with 1350 joints, very heavy, but so far working great, and tested to around 8000rpm with no problems.

 

Carbon fibre shafts are great for critical speed issues. (but not so hot for critical $$ ones...)

 

One of the biggest benefits of dry sump lubrication is that durring High G cornering Acceleration and similar motions, you will get no oil starvation due to the oil moving away from the single pick up point of a standard Wet sump, which is why the systems are primarily used in races with corners like F1 and The Lemans series cars.

It is probably not as beneficial in drag racing except for the added benefits listed above, but usualy a race dry sump system will have 4-5 stages or more, which adds to parasitic loss to the drivetrain. Just for example the C6 Z06 uses a two stage Dry sump, (one pressure stage and one scavange stage) A Street Ferrari will use a 3-4 stage (one pressure with two to three scavange stages) Each scavange stage will pick up from a different location on the sump, this helps to make sure that the oil is scavanged and sent back to the resevoir from any location it may be sitting at.

One Other added benefit of the Dry sump system is the ability to add an inline Oil cooler for the engine oil to keep it at the optimal oil temperature, this acts on top of the oil cooling effect that a remotely mounted sump has.

 

You can add an oil cooler to any installation. Doesnt have to be a dry sump setup.