I saw a top gear episode a while back that claimed an M3 got better mpg around their test track then a prius. I wonder if it's because the M3 was drafting off the prius. Also they make a claim at the start of the video about the amount of environmental damage cause from making the batteries. How much CO2 is produced in making a prius battery?

here is the video...

http://www.youtube.com/watch?v=dKTOyiKLARk

The whole point of the segment was stating that how you drive effects the fuel consumption more hat the type of car. although this can only be taken so far, I'm sure they won't be able to repeat the results with a hummer.

There may be a point with the drafting,

The hybrid advantage is also limited on the topgear circuit as its a high speed tract so brakes aren't used too much.

The point was that cars,if driven outside of their limits will get poor mileage.

A Prius running at its top speed is not going to get the same mileage as a sports car that is cruising along at a average clip for it's design.

First of all breaks are used a lot on any race track. you have to break when making most of those turns and you are breaking hard. I know people don't drive around race tracks all day but people also don't drive 55 and stay in one lane while on the high way. Lets say you are driving between 55-70mph moving in and out of traffic which car will get better gas mileage?

I guess i agree with wv_engineer and the real myth would be that if you are an aggressive drive a hybrid will give you less mph then a traditional sports car.

It's not even really aggressive driving. The Prius is a small engine car that is designed to maximize fuel use during slow to medium speed stop an go traffic. I didn't think you could be an aggressive driver in a Prius.

Once you get into a long haul on the s freeway where you are not using your breaks regularly, the battery pack and hybrid system is not only useless, it is counterproductive. The hybrid system is just dead weight. Without the hybrid system, the cars lack of power starts to show. in order to maintain a high speed without battery assistance, the engine has to run full out. this dramatically drops the fuel economy of that car.

Where as the M3 is a sports car that is design for long distance high speed. Such a drive is well with in it's design parameters for efficiency at a speed that a Prius struggles with.

The two cars are designed and optimized completely different types of driving.

By using a car in a condition that it isn't optimized for, then the car suffers.

quote:

Originally posted by andcbii:
First of all breaks are used a lot on any race track. you have to break when making most of those turns and you are breaking hard. I know people don't drive around race tracks all day but people also don't drive 55 and stay in one lane while on the high way. Lets say you are driving between 55-70mph moving in and out of traffic which car will get better gas mileage?

I guess i agree with wv_engineer and the real myth would be that if you are an aggressive drive a hybrid will give you less mph then a traditional sports car.

I think you mean brakes. Not breaks. Generally if you break during a race you lose.

quote:

Originally posted by wv_engineer:
Without the hybrid system, the cars lack of power starts to show. in order to maintain a high speed without battery assistance, the engine has to run full out. this dramatically drops the fuel economy of that car.

No. For a given power requirement, running a small engine at full load is more efficient than running a large engine at part load.

An important thing to note is that the Prius is scrubbing speed off on every corner while the BMW is able to follow quite easily because of its better tires and suspension.

quote:

No. For a given power requirement, running a small engine at full load is more efficient than running a large engine at part load.

Engine, yes, but only the engine. You are ignoring the whole rest of the drive train and how it plays into the dynamics of the car.

With the extra wind load and rolling resistance, etc., the engine may be operating at it's most efficient point the the whole car as a system is not.

quote:

Originally posted by srmarti:

quote:

Originally posted by andcbii:
First of all breaks are used a lot on any race track. you have to break when making most of those turns and you are breaking hard. I know people don't drive around race tracks all day but people also don't drive 55 and stay in one lane while on the high way. Lets say you are driving between 55-70mph moving in and out of traffic which car will get better gas mileage?

I guess i agree with wv_engineer and the real myth would be that if you are an aggressive drive a hybrid will give you less mph then a traditional sports car.

I think you mean brakes. Not breaks. Generally if you break during a race you lose.

quote:

No. For a given power requirement, running a small engine at full load is more efficient than running a large engine at part load.

Actually, this is not the case.

Car engines get maximum torque at a different running speed than they get their maximum power. This is due to how the flame velocity, piston speed, compression, pumping losses, valve timings, ignition advance etc. play in the engine.

An engine is the most efficient when it produces maximum torque, e.g. when the most heat in the cylinder translates into pressure on the piston that translates into a cranking action for the longest possible time during the stroke. However, power continues to increase as we move out of that optimum zone because power is a product of speed and torque, and the speed increases faster than the torque drops until eventually the torque starts to drop faster and losses grow until the power peak happens and then things start to go downhill.

Almost all engines have their efficiency peak between 1/2 and 3/4 of the speed where they get their maximum output. The Prius Atkinson cycle engine is also optimized for a rather narrow range of running speeds, which compounds this effect. When you put your foot down, it slips past efficiency into power and the mileage drops dramatically.

Under normal operation, the Prius hybrid drive is designed to keep the engine in the sweetspot by acting as a constantly variable transmission. It actually circulates power back through the system constantly to detach the engine speed from the road speed at a loss of some 30%, but as the engine is doing most of the work directly, this loss is neglible enough to allow the system to operate at a higher efficiency than it would should the engine be forced to run at a fixed speed ratio to the wheels.

It's a sort of hybrid-hybrid - half way between a parallel and a series hybrid. It gets worse efficiency than pure parallel or pure series hybrids, but this compromize allows it to work better in situations where parallel or series modes lose their advantages.

quote:

Originally posted by wv_engineer:

quote:

No. For a given power requirement, running a small engine at full load is more efficient than running a large engine at part load.

Engine, yes, but only the engine. You are ignoring the whole rest of the drive train and how it plays into the dynamics of the car.

With the extra wind load and rolling resistance, etc., the engine may be operating at it's most efficient point the the whole car as a system is not.

Your argument about higher speeds actually weakens your case.

For a given distance and a given time, both cars will have the same average speed but the slower accelerating car will have to reach a higher peak speed to keep up, and will experience higher aero and road drag as a result. Accelerating at a higher rate is more efficient and that fact has been proven many times.

I didn't get everything in on that last post.

If you have a specific example of how you think I am ignoring the rest of the drivetrain, please post that and I'll reply to it. As it turns out, it appears to me that you are ignoring the dynamics of the rest of the car in the Prius vs BMW issue. Tire scrub is the major difference in efficiency between the two cars.

quote:

Originally posted by einomies:

quote:

No. For a given power requirement, running a small engine at full load is more efficient than running a large engine at part load.

Actually, this is not the case.

Actually, it most certainly is the case. It appears that your knowledge of engine theory and torque vs hp (a sticking point for most people) is good but you seem to have missed out on the part about load and have substituted an argument about rpm and torque peak. Do some reading about BSFC and we'll talk. Here's a good article:

http://www.autospeed.com/cms/A_110216/article.html

I certainly agree with most of your post but there are a few things that would warrant further discussion. "Pumping losses" is a commonly misused phrase for one.

quote:

Originally posted by einomies:
An engine is the most efficient when it produces maximum torque, e.g. when the most heat in the cylinder translates into pressure on the piston that translates into a cranking action for the longest possible time during the stroke.

Correct. Now, can you tell me why an engine is usually (but not always) most efficient at the torque peak?

quote:

Correct. Now, can you tell me why an engine is usually (but not always) most efficient at the torque peak?

I think it is due to the speed of the piston in relation to the speed of the combustion. If the piston is too fast, the mixture expands too fast and the average temperature remains lower, therefore producing less pressure. Then, according to Carnot, the efficiency should be lower.

If the piston is too slow, the pressure builds up, the temperature goes up higher and more heat is lost through the cylinder walls and head. There' is a sweet spot between the two. This is a very simplified explaination and there are a lot more factors at play.

If you're insinuating towards to your argument about throttle position, I might add that at any throttle position, the engine should be most efficient at the running speed where it produces the maximum torque.

My point is, that this running speed isn't where the engine can produce its maximum power. The difference here is that you consider "full load" to be a WOT situation at whatever speed the engine is running at the moment, and I consider "full load" to mean the maximum power the engine can put out at any running speed. An engine is certainly not the most efficient at peak power.

If you search for a chart about the Prius Gen II engine, you will notice that it gets the efficiency peak at 1/2 of the maximum power at around half the running speed from the power peak. This remains true regardless of the throttle position.

To drive fast in both the Prius and the M3 requires that you drive the Prius flat out which decreases its efficiency significantly, whereas the M3 only needs a part of its engine power for the same speed and that makes it, if not more, then at least similiar in efficiency. It's just a matter of choosing the right gears to keep the engine in the sweet spot.

Did you read the article? It explains things quite well and gets most things right.

When discussing engine performance, the load on an engine is typically denoted as the percentage of the maximum torque that the engine can produce at any given rpm and is not generally considered to be related to the hp. I can see why we haven't quite lined up on this even though most of the other parts of our posts jive.

The piston speed has very little to do with where the torque peak of the engine occurs. After about 3000 rpm, you'll notice that most engines no longer require a spark advance curve because the increase in combustion speed that occurs essentially tracks the increase in engine and thus piston speed. 18,000 rpm poses very little trouble for the combustion event; however, feeding enough air through the engine at that speed is a major issue (as well as keeping the engine components from committing spontaneous disassembly ).

So what does that mean? It means that volumetric efficiency is the main factor that determines where the engine produces the highest BMEP and therefore torque. On the low side of that point, heat can be lost to the internal engine components but that is usually a small effect. In racing engines, the lengths of the intake and exhaust runners has a huge effect on the torque output, and even in our lowly production engines those runners respond to inertial tuning by resonating at a particular engine speed and enhancing the air flow through the engine, increasing the volumetric efficiency and boosting the torque output at that rpm. Below and above that point, the air flow and thus the torque will be lower. As you noted, as the engine rpm continues to climb and the airflow starts dropping off quickly, the engine will be close to the hp peak and things will go downhill from there.

On the BMW vs Prius test, the Prius is undoubtedly not at its peak efficiency but it isn't all that far off, most likely not more than 20%, while the BMW will also be off by at least that much if not more because of the low engine demand (low load). The BMW also outweighs the Prius but manages to overcome that disadvantage too.

The main difference is that the Prius is scrubbing off speed just like the driver was dragging the brakes continually, while the BMW because of its better tires and suspension experiences much less of that. That's the main difference between the two, not the engine efficiency. The Prius has to use more fuel to counter the effects of cornering at the limit, while the BMW loafs along behind.

Simple physics and pretty typical of the difference between a regular car and a high performance car. Setting up the Prius properly with better tires and suspension to match the capabilities of the BMW would change the outcome dramatically.