Pulse&Glide vs. Constant speed?

Hi SageBrush,
I'm sorry, :embarassed: I wasn't clear. I meant that pumping power here is only 1.3% of the power represented by the total amount of gasoline burned to produce 7.5KW (my 10 HP) of engine power.
My point, again, is that pumping losses are only a small contributor to low power inefficiency. The percent of power in the gasoline that can't be utilized by the engine goes from about 65% to over 80% from optimum engine efficiency to near idle. The additional 1.3% from pumping is not what counts.
-mort

 

Hi Tom,

You may think this is a quibble,hwell: I don't agree that if the car isn't moving then the engine isn't producing power. Even at idle the engine is producing shaft power. It all goes to ancillary uses, like oil pumps and windage in the gears. And since we're talking about the Prius, the battery might be charging. Even then the pumping losses are under 5% of that power.
-mort

 

Hi Mort,

The (negative) 25 - 50,000 Pa manifold pressures at 60 mph are Prius values, are they not ? How much lower are they in say a typical 3L Otto engined car ?

Anyway, I come up with about 3.5% throttle losses at 60 mph going 60 mpg, about what you calculated too I think. How about 30 mph ? If I am thinking about this right, then the throttle power consumption stays about the same but fuel usage goes down to 2/3rds or so, implying that the fractional power loss goes up to about 5.3%.

 

Hi SageBrush,
You are right, pumping losses for partial throttle don't change much in a conventional Otto cycle engine. As pressure drop across the throttle increases the quantity of air falls. For the Prius, the transmission will load the engine for near wide open thottle under most conditions. But in a conventional car you are either in a different gear at 30 than at 60 or else the engine will be running at half the rpm. Either way you can expect the throttle to be in a different place.

So the speed is immaterial, 60 mph is just a nice round number. The significant variable is power. I picked 10 HP because there is pretty good evidence that the Prius uses about 10 HP to maintain cruise at about 50 to 60 mph. 10 HP, 7500 watts, is already pretty far down the power curve (about a tenth of the maximum that the engine can make). You can do the math for 5KW or any amount you want. That graph of the 2ZR-FXE shows what fuel consumption you could assume at WOT, so efficiency would be lower at partial throttle. You'll have to make up your own pressure drop, I haven't seen any published data for the Prius.

You asked about assuming 25 - 50,000 Pa manifold pressures. I pulled that out my nice person. Fuzzy1 (Thanks!) gave us some real numbers for cruise.

My point, again, is that pumping losses... :blah:
-mort

 

Okay, I see where you are coming from: the 5% losses are small when compared to total losses. This is where it gets interesting, since most of the losses in a modern gasoline engine are unavoidable with today's technology.

First you have losses do to thermodynamics. For work to be done, heat must flow from a higher temperature source to a lower temperature sink. In the case of a gasoline internal combustion engine, the high temperature is inside the cylinder, the low temperature sink is the ambient air. The theoretical maximum efficiency for any given temperature delta can be determine by looking at a the theoretical Carnot cycle engine. That's as good as it can get, and nothing you can do will improve the theoretical efficiency short of a hotter engine or colder ambient air. More information can be found here: [ame=http://en.wikipedia.org/wiki/Carnot_cycle]Carnot cycle - Wikipedia, the free encyclopedia[/ame]

The thermal losses of a real engine are by far worse than the theoretical Carnot cycle. Real engines require cooling to keep from melting. All of this cooling wastes energy. It is possible to design an engine that works without cooling, but perhaps not practically with present materials. This type of engine is called an adiabatic engine: Technology Review: A More Efficient Engine

Given that we are stuck with these losses, it would be more informative to measure pumping losses against the other remaining controllable losses. That way we could see how much contribution it makes to avoidable losses. Otherwise it's a lot like a budget discussion where all of the major items are fixed (housing, basic food, etc.). The things you can control appear too small to be of interest, even though they are all you can control.

Tom

 

Not to hijack a thread that is already in need of help - but...

We have already established many 2010 anomalies that account for crazy things that in the physics world do not hold up - yet have been proven - these include the Mud flaps increase mileage - 3.14%.

Carnot engines are a common way to state things like temp in not equal to temp out - means lost energy.

I think the bigger picture should be concentrating on things that actually increase mileage (from memory - mud flaps, antenna, and wheel covers {removed})...

Just a thought...

 

Hi Tom,
That HCCI engine is, I suppose, a wonder. Smokey Yunick worked on a similar idea for about 50 years. I never thought Smokey understood the themodynamics, but maybe the boys at MIT know better.

Right. For the Prius, Atkinson type engine with a CVT, the pumping losses are about as contained as they can be. Low power is inefficient primarily because the sparse charge delivers a lower peak temperature. The charge is sparse because a reasonable engine displacement is required for acceleration and hill climbing.

So you can either not use the engine in an inefficient range. For instance, the original idea behind the Chevy Volt eliminates low power losses in the ICE. Or you can improve the engine. I saw here that Saab has a variable compression ratio design. A variable displacement engine, if it had a range of a factor of 10, could improve low power efficiency. I suspect any design would be horribly complex. However, a car with 2 engines...

-mort