That's all. Specifically, how eliminating a throttle body reduces pumping loss.

Thanks,

Todd

 

How does one control the engine if the throttling device (TB in this case) is eliminated? Did you mean something else?

 

No, I didn't. I often hear that technologies such as BMW's Valvetronic, which eliminates the throttle body and uses the intake valves as the throttle, or diesels are more efficient because of reduced pumping losses from eliminating the throttle valve.

I'm having trouble grasping this, conceptually. Trying to figure it out.

 

In the case of diesels, they control engine speed by adding fuel. Lean mixtures are not destructive in diesels, so they are speed limited by fuel & have no throttle plates. This means there is no restriction to air flow & the engine does not have to work to get air as one would through closed throttle plates.

For example, a gas motor cruising down the highway is drawing 15" vacuum. It is trying to spin faster, but cannot, so it exerts energy against itself, hurting efficiency. It is making more power than it is using to propel the car. This is why big engines typically use more fuel than a smaller engine in the same car.

Variable valve timing engines can alter the cam duration & phasing for optimum efficiency at different speeds to help limit pumping losses. These still restrict the engine via the valve, so I tend to believe this is still far from the efficiency of diesels, but could be a definite improvement over a throttled engine.

 

See, though, your explanation is exactly backwards of what I'm thinking. Example-

I just did an experiment with my house vacuum. With it on and sucking on the hose, it draws 815watts (I have a wattmeter). If I put my hand over the hose, the power draw is REDUCED to 730watts. Why? Because with my "throttle" in place, the amount of air moving through the vacuum cleaner is reduced, and therefore the motor is doing less work.

I cannot see how these rules change for engines. My vacuum is a centrifugal pump, and an engine is a positive displacement pump, but that doesn't change the rules.

Here's what I posted over in the Auto News section-

 

My SWAG at it:

To measure pumpiing loss you need to pull the same amount of air through the restriction that you did without the restriction. That will require more energy input to the motor, and that additional energy is the "pumping loss."

 

The very definition of a throttle means that you pull less air.

 

Specific to this topic, pumping losses are those inefficiencies associated with having a large pressure differential across the throttle blades—vacuum. Whenever there is a large vacuum, such as light load cruising and idle, the engine is having to work more to operate the piston in a cylinder under vacuum. The fuel and air required to operate a throttled engine are thus greater than the same engine using a non-throttled technology, such as said valvetronic solution from BMW because the engine is not having to work against an entire intake system under negative pressure, only the cylinder. As a result, efficiency increases, among other changes.

So, how does BMW do this? Yes, the timing is variable, but your concern with throttling is chiefly coordinated by the variable lift technology, which is 0-9.1mm. This might seem impressive, but the manufacturing precision they are boasting is extraordinary, finish tolerances in the neighborhood of hundredths of a millimeter (I have not verified this myself); think about that next time you let your main bearing clearances by with a thousandth of an inch deviation!

In case you weren't able to figure it out from the above, this technology does not eliminate all throttles because the intake valve(s) itself becomes the throttle for each cylinder. Yes, they have rid the engine of a common throttle to all cylinders, but the underlying principles of stoich (or near, in comparison to diesels) combustion for gasoline engines has remained.

 

I would think to make that a fair comparison you need to change your experiment a bit. Just plugging the vacuum is going to result in less work and less power draw.

An engine is going to need to pull enough air to mix with the correct amount of fuel to make so much power. If a car needs to make 20 hp to maintain a certain speed on the highway, it's going to need to pull so much air in order to produce that much power.

To make this fair, put an MAF sensor on the vacuum, a speed control on the motor, and a restrictor on the vacuum tube.

Try to maintain 20 g/s of airflow without the restrictor by adjusting the speed of the motor. Look at the power draw. Now install the restrictor and repeat. To get the same amount of airflow you will need more power, whether that be from fuel or electricity.

It's simply work to draw air through a restriction. It takes energy to create the vacuum in what would otherwise be at atmospheric pressure. The air temperature and pressure drop, that takes energy. That energy has to come from somewhere. It's a potential energy that the engine has created, luckily for power brakes. There is work performed in not only moving the air but also by changing its current state.

It does raise another good point though. A diesel engine draws a larger amount of air than is needed for combustion. It doesn't have the reduced efficiency of a throttle however it will perform more work to move all that air. I'm sure it does more pumping work but at greater efficiency. I think the biggest advantage of a diesel comes through the improved thermodynamic efficiency from higher cylinder pressures. More thermal energy is converted to kinetic energy rather than spewing out the exhaust and radiator.

I'm not sure how correct my statements are but this is how I try to see it. Even these simple things can give you a headache, huh? I do know that the experiment is always right and restriction does indeed cause more pumping losses. Trying to make physical sense of all that is more difficult than simply knowing it!

 

Another thing to think about is the energy imparted into the airflow. Gasses are notoriously inefficient to pump. It takes quite a bit of energy to move so much mass of gas compared to that of a liquid. Those pesky little gas molecules have a mind of their own!

What happens to the temperature of the exhaust on the vacuum when you plug it up? Gets hot doesn't it? I'm sure part of that is the motor's thermal energy being absorbed by less mass flow but I'm sure some of it is generated by the friction of the air molecules.

Say you ran the above experiment again but also measured the temperature of the exhaust gasses. I'm willing to bet you'd find the exhaust of the restricted experiment to be of higher temperature than the unrestricted. I'm sure the exhaust of an engine being driven on a spintron is not exactly cold either even though there is no combustion.

If you want to set your mind at ease as to where that energy "disappears" to, I'll bet a lot of it goes out the exhaust pipe in the form of higher temperature exhaust gasses.

Still trying to think of good examples to illustrate, hopefully none of them make me appear to be too stupid

 

Not a valid experiment, IMO. If you compare two engines, one gas and one diesel, running at the same RPM, the diesel will be said to be more efficient, partly because of reduced pumping losses.

Anyone know the differences in cam timing-specifically intake valve closing events- between a typical gas and diesel engine? Let's say the 6.0 vs. the Duramax.

 

You were speaking of pumping losses. Throttles, dirty air filters, clean air filters, bends, and any other thing interacting with the flow incurs a pumping loss. The experiment is completely valid. Everything else is held equal except for the restriction change. A throttle is nothing more than a controlled restriction, it's the definition of a throttle. Why do you want to introduce as many other variables as possible into your experiment?

You asked "That's all. Specifically, how eliminating a throttle body reduces pumping loss." If a throttle body wasn't a pumping loss it wouldn't work to throttle an engine!

Diesel engines often have comparatively massive intake, exhaust, and no throttle. As I said before a diesel engine moves more air than is needed for combustion so more work is done in moving air but it does that work more efficiently. I couldn't tell you the net effect nor do I really care. Good luck on finding exacting estimates of the energy used for each for a specific engine in a specific application during specific conditions.

You might want to ask yourself why cylinder deactivation works to improve fuel economy in a gasoline engine. It works for many of the same reasons that a diesel engine is more efficient at part throttle than a gasoline engine.

I don't understand what more you need to know? Aren't you an engineer?

 

dont forget diesel fuel in general contains more energy than gasoline therefore you can use less to do the same amount of work when compared to gasoline

i believe straight gasoline contains about 125,000 btu's per gallon worth of energy blended with 15 percent ethanol its probably around 118,000 btu's

comparatively diesel contains about 138,000 btu's so it will require less to do the same amount of work. when you take this into account the pumping losses between a diesel and gas motor are rather insignificant

think about it when you have vacuum in your intake manifold when the intake valve opens it must work against a vacuum but on the compression stroke it requires little work to compress the little gas thats there in a diesel there isn't a vacuum to work against with the intake valve opens but then you must compress those gases to several hundred psi

 

I have explained precisely why removing a throttle common to all cylinder will decrease pumping losses.

Do you still not understand why this is so (such as BMW's engines or a diesel)?

 

Precisely? C'mon.
I do not think this explanation is correct- perhaps so in the very vaguest sense, but certainly not a complete explanation. If that were true on the face of it, then cylinder deactivation tech, where the cylinder is completely closed off, would not net anything.