Inliners International Forums General Discussion Drive Train Overdrive?

Glad to hear it is a 3.23 and you have a tach. A 27" tire will help lower rpms and not be out of sorts because it is too big. Mine are that big.
Your highway mileage will certainly be better than city driving.

http://www.mechadyne-int.com/vva-reference/part-load-pumping-losses-si-engine

Look at the P-V diagram in Figure 2. The difference between the power loop area and pumping loop area is the engine power output. Greater throttle open at 2200 rpm compared to smaller throttle opening at 3000 will result in less pumping loss (smaller pumping loop on the diagram). Granted, there is more to the story (fuel and spark curves and mechanical losses for example) but this is the major reason OD helps mileage. It's also the major reason diesel engines get better mileage. Diesels have very small pumping loops at all operating conditions because they are not throttled. It is also the major reason small engines get better mileage than large ones. The small engine will require greater throttle opening and therefore have less pumping loss.

If you are having difficulty with the pumping loss concept, consider that pressure in the crankcase is 15psi or 1 atmosphere while pressure in your intake manifold is much lower at part throttle, let's say it's 5psi. The piston has to push against a 10 psi differential on the intake stroke. This is pumping loss.

Have you ever noticed how letting off the gas doesn't slow a diesel (without a jake brake) vehicle as much as it does a gasoline? That's becasue the pumping loss is much greater in the gasoline engine.

I feel the 16 to 22 :1 compression ratios (P in the formula)is the diesel's greatest forte.

- now are we speaking of four cycle engines or two cycle types? All the loop talk reminds me of two stroke engine talk where you have no exhaust or intake valves just ports or slots in the cylinder bores to allow intake charge from the somewhat pressurized crankcase and exhaust passage from the upper cylinder area after firing. I think this is what a two stroke detroit diesel does or is.



I can see atmosphere pressure in the crankcase , I am at a loss as to why there is 5 psi (should that be 5" of mercury vacumm?) in the intake. How can you have 5 PSI in the intake manifold and still be naturally aspirated (non-blown) on a four stroke gasoline engine?

Its over my head--duck here it comes again!

Four stroke. True, higher compression helps the diesel but probably less than reduced pumping loss.

Intake manifold vacuum is relative to atmosphere or "gage" pressure. Intake manifold pressure is relative to zero or "absolute" pressure. Really the same thing, just less confusing to do calculations if you use absolute pressure instead of manifold vacuum.

Maybe it's helpful to think of the Fig2 P-V diagram this way: A diesel engine is unthrottled and always has a small pumping loop area. Load control is achieved by underfueling which makes the power loop area smaller to match the load.

A gasoline engine at WOT is unthrottled and also has a small pumping loop. But when throttled for load control the pumping loop gets bigger and the power loop gets smaller to match the load. This bigger pumping loop for a throttled gasoline engine is a major power loss at cruise. Really nothing you can do about it except to get a smaller engine or overdrive.

I don't agree with you panic.

Pressure on top of piston on intake stroke is manifold pressure while pressure below piston is crankcase pressure. The difference results in pumping loss at the crankshaft. You are thinking too deep. Go back to the basic theory. P-V diagram.

Greetings . . .

I am with strokersix on this one. An engine is first and foremost an air pump. Where there is pressure differential there is work to be done. Part throttle conditions create vacuum because of atmospheric pressure difference between the intake plenum and the crankcase. This creates pumping loss which uses fuel when cruising.

The crankcase doesn't need to be sealed in order to be under pressure. The weight of the earth's atmosphere (which varies a little depending on where you are with respect to sea level) is always there. Moreover, a vacuum in an intake manifold is still a positive pressure when compared to the true vacuum of outer space! What matters is the difference.

An overdrive unit reduces RPM and therefore the amount of air passing through the pump over a give period of time. The objective is to cruise by applying just enough fuel to do the work of maintaining speed.

Jets and circuits in a carburetor 'weep' fuel into a vacuum and abate as pressure equalizes. With a carburetor the throttle plate position is controlled by the drivers foot. In a modern engine the plate is controlled by a computer with input from a mass airflow sensor.

Cruising at low throttle openings is to be in too low of a gear and therefore at to high RPM.

I vote for the OD.

regards,
stock49

Have a nice day.

While an engine might be described as an air pump, it's source for power is gasoline. Before gasoline can be used as fuel for an engine, it must be atomized so that it can be mixed with air to form a combustable mixture. Contrary to popular belief, gasoline in its liquid state is not combustable; only gasoline vapor will burn. Vaporization is the act of changing from a liquid state to a gasseous state and only occurs when the liquid absorbs enough heat to boil. The temperature-pressure relationship dictates that as pressure is reduced, the boiling point is also reduced.

Since the pressure in the intake manifold is far less than atmosphere, the boiling point of the gasoline is lowered considerably. At this reduced pressure, latent heat absorbed from the many air particles surrounding each fuel particle causes some vaporization, which is further aided by heat on the intake manifold floor.

Because complete fuel vaporization is the result of many factors (ambient temperature, fuel temperature, manifold vacuum, and intake manifold temperature), it is easy to see that anything which reduces any one of these factors will adversely affect vaporization and thus reduce fuel economy (and increase exhaust emissions). Some examples would be cold weather, an inoperative exhaust heat control valve, and high overlap camshafts and/or heavy throttle demands.

While the effects from lower temperatures are obvious, reduction of manifold vacuum either by valve timing or heavy throttle operation are highly detrimental to fuel economy due to the higher pressures (and boiling points) resulting in the intake manifold which reduces the amount of fuel vaporization which will occur by the time the charge enters the combustion chamber. Fuel not vaporized at the time of induction, is to a large extent, exhausted unburned from the combustion chamber ( and can cause high hydro-carbon exhaust emissions).

I am having a hard time with this crankcase pressure stuff...isn't the crankcase pressure dealt with by either a road draft tube or better yet the Positive Crankcase Ventilation system? A properly working PCV system "reclaims" unburned blow-by gasses by returning them to the intake manifold and, this being a combustable mixture, is added to the new air-fuel charge. Wouldn't the increased gasoline economy of the PCV system at least counteract the crankcase pumping losses?

Fasteddie, Crankcase pressure is assumed equal to atmospheric pressure, nominally 15psi. Sure, the deeper you look it's more complicated as panic suggests but the basic assumption is valid.

PCV won't change the situation much because it is a mixture of air and fumes but EGR (exhaust gas recirculation) will because it's inert (composition nitrogen and combustion products). Including inert exhaust gas with the fresh charge increases the inlet manifold pressure and reduces pumping loss. There are some other benefits to EGR as well. It's not a bad thing, contrary to popular belief. EGR increases mixture motion in the combustion chamber because there is more of it and provides heat for vaporization. On the negative side EGR reduces peak temperatures which reduces nitrogen oxides which is the reason for EGR. Properly functioning EGR will help fuel economy and won't hurt power because at WOT EGR shuts off.

Why wouldn't PCV change the situation much? You are removing some of the pressure from under the piston. It seems it would be at least as effective as EGR because EGR is putting UNCOMBUSTABLE exhaust into the combustion chamber taking up space where COMBUSTABLE air and fuel could reside thus effectively diluting the intake charge. PCV is displacing combustable intake charge with COMBUSTABLE mixture.

On a more basic level, wouldn't the atmospheric pressure under descending piston A be inversely proportional to the atmospheric pressure under ascending piston B in the same engine?? The pressure trying to compress under piston A due to its downward travel would simply fill the void left by piston B's upward travel thus netting no pressure increase in the crankcase?

PCV won't change the pumping loss because it is a combustible mixture. PCV flow must be accompanied by reduced flow through the carb. EGR does change the pumping work loss because it is inert. Adding inert exhaust gas does not change the amount of new mixture through the carb so you get a net increase in intake manifold pressure. Increased intake pressure translates directly to reduced pumping loss.

Evacuating the crankcase doesn't help because you pay an offsetting penalty on the exhaust stroke. We've been talking about intake stroke because the intake is throttled but the pumping work argument applies to the exhaust stroke as well. Pumping work on the intake stroke is a loss proportional to the difference between crankcase pressure and intake manifold pressure. Likewise exhaust pumping work is proportional to the difference between exhaust manifold pressure and crankcase pressure. Fortunately the exhaust manifold isn't throttled so this loss is small. If you evacuate the crankcase it helps intake pumping but hurts exhaust pumping in an equal trade off.

Fig2 on this link can be used to understand all this discussion and more:

http://www.mechadyne-int.com/vva-reference/part-load-pumping-losses-si-engine

Supercharging, turbocharging, valve event timing, pretty much every aspect of engine operation affects the shape of this P-V diagram. It's just a tool to understand what's going on inside the engine.

For the descending piston A versus B yes, you are correct. This is why the crankcase pressure is roughly constant at 15psi or one atmosphere absolute pressure. Crankcase gases pushing from under piston A to piston B and spinning crankshaft drag are additional losses above and beyond the pumping loss I'm talking about.

O.K. we are getting nowhere....obviously what I am calling "pumping losses" and what you are calling "pumping losses" are not the same thing. Define "pumping losses".

OK let's try this:

We all can agree that the pressure above the piston on the power stroke is greater than the pressure above the piston on the compression stroke and is a net positive. This is what we're after, positive work.

Now think about the intake and exhaust strokes. The pressure above the piston on the exhaust stroke is greater than the pressure above the piston on the intake stroke. This is what we have to pay to transfer gasses out and in the cylinder, negative work. The term used is pumping work. Pumping work is quite large when the pressure above the piston on the intake stroke is low in a throttled condition.

The power loop of fig2 is compression and power strokes while the pumping loop is the exhaust and intake strokes. Areas inside loops are work. Power loop is positive work, pumping loop is negative.

Crankcase pressure cancels out of all four strokes because it is assumed constant.

Help any?

Greetings . . .

Here's a link that I found really useful in getting one's head around pumping loss:
StraightDope
I am big fan of practical stories.

And here's some links showing empirical data that lower RPM is the place to be:
TDIclub
MetroMPG

This is where I am coming from when I vote for lowest possible RPM and a decent throttle position equals best economy. As the desired cruising speed rises the need for OD becomes evident.

regards,
stock49

You guys are a long ways from Louisiana to be practicing such VOODOO!

Remember Diet Smith in the Dick Tracy comics, a few decades ago? He used to say "He who controls magnetism, controls the world"

Snowman are you getting all this? I just took the '53 for a drive after the long winter. I was afraid to shift into 5th gear for fear of loosing power, wasting fuel, or something worse. Beater