Engines.. Apples To Apples....

[ontarian_frog]

Well let me tell you that the front page article at
www.autospeed.com called The Real Way Of Comparing Engine Designs was very interesting to say the least. Albeit a little hard to understand if you have limited knowledge of engines in general.

In general, the article mentions that you can compare engines, apples to apples, by calculating the BMEP. The BMEP is the effective (actual) pressure on the pistons. The higher the pressure, the better the engine (in general).

But what the Author fails to mention, is that if you compare the Effective Pressure (BMEP) and the static pressure, you can tell how good the engineers are at their jobs. If you get a BMEP higher than the static compression ratio, it is more than 100% efficient at filling the cylinder with air.

Here are the major sub-compact results:

1st gen Aveo: 9.6 (77kw @ 6000rpm, 1.6L, 9.5:1)
2nd gen Aveo: 9.5 (81kw @ 6400rpm 1.6L, 10.8:1)
2010 Yaris: 10.5 (79kw @ 6000rpm 1.5L, 10.5:1)
2010 Fit: 10.9 (89kw @ 6400rpm 1.5L, 10.4:1)
2010 Versa: 10 (80kw @ 6000rpm 1.6L, 9.8:1)
2010 Accent and Rio: 10.3 (82kw @ 6000rpm, 1.6L, 10:1)

Yep, you read right! The new Aveo engine is the only one that doesn't get 100% cylinder fill. But what does it mean? Unfortunately, the only thing I can think of is that the intake system isn't tuned for maximum cylinder fill at maximum power. So lets do the math:

At maximum torque, 142Nm @ 3800rpm, the engine makes 56.5kw. That gives us an 11.2 bar result. Meaning that the engine does get over 100% cylinder fill, just not at maximum power.

Lets look at the 1st gen Aveo: It gets 145Nm @ 3600rpm (54.7kw). You'd think it would do better than the 2nd gen. And it does. The result is 11.4. Now what?

Well it's easy to see that the new ECOTEC...is just plain... good marketing. In people's eyes, on paper, it looks better. Sure, makes more power. It's named after the famous Ecotec that saved the Cavalier. It uses more advanced technologies like variable valve timing. But in the end, the e-tec is simply better.

Well, I'm not sure what to think. The newer Aveo's get better gas mileage, but they also have better aerodynamics. Is the new engine really better on gas? I don't think so. I think if someone was to put one in a 1st gen body, we'd see bad results. Maybe they put more time on the fuel and ignition maps and the combination of better aerodynamics and software works together.

What do you guys think?

[Thymeclock]

What do you guys think?

I think all these engines are relatively comparable and that the fuel economy has less to do with the engine design than with the transmission design, if it is an automatic. My 2009 Aveo gets lousy FE in city driving (at any speed under 45 MPH), 22 MPG at most. But once the torque converter locks up at 45 MPH it gets 37 MPG. It's a four speed automatic and the brilliant engineers at Chevy programmed it to have the same delayed shift points as all the other gas hogs they produce. That gives the Aveo decent acceleration in the lower gears, but at the price of poorer fuel economy.

Chevy pitched this car to rural America with a high highway MPG, and it really does achieve it at highway speeds - but if you are a city dweller who does no highway driving the local fuel economy sucks, especially for such a small car. Honda and Toyota apparently are using a more sophisticated transmission (probably a 5 speed?), or at least one that is programmed to lock up much sooner than 45 MPH. The Versa and the Rio are about the same as the Aveo when it comes to relatively poor MPG.

[petrified.rabbit]

Is the first gen compression ratio versus the second really that far apart? is compression ratio important is his calculation? this is an interesting way to look at it, using his method though i have some questions... i'm not sure what part of the engine he is comparing? technology? looking at only peak hp at its specific rpm, is it sustainable? i know honda can run at 7400 rpm, but are you going to be at 7400 rpm all the time? not that comparing different engines at the same rpm is the solution, but if you could add peak torque at its rpm, and number of cylinders to that equation, i think it would make truer sense of engines. not sure how to do it, mathematically speaking..

i also cannot make sense of what the 1200 multiplier stands for, how did he come up with that number?


but using his equation:

hayabusa :

1.3l with 145 kw at 9500 rpm = 16.03 bmep

an audi tt 225 engine:

1.8l with 165kw at 5900 rpm = 18.64 bmep

vxr corsa:

1.6l with 141kw at 5850 rpm = 18.07 bmep

i put the vxr here because according to my research the ecotec aveo uses the same block, and head. its just a detuned engine.. non turbo, naturally aspirated us emissions design...

the new aveo versus the old is ecotec: 105 ft/lbs @ 3800 rpm and the etecII: 107 ft/lbs @ 3600 rpm so the etec is a slightly torquey motor compared to the ecotec, but with less peak hp.. i believe (not sure honestly) with the variable vale timing change, that the ecotec has more hp and torque through the power curve overall?

i dont think either engine is better than the other.. and that most of the fuel economy is from a slight change in gearing..

[thehunterooo]

i think the 2nd gens peak at 103hp as well

weight and tires/sizes also effect mpg ratings as well

[ontarian_frog]

BMEP is the actual average pressure that pushes on the pistons. The higher the number, the more optimized the engine is. It is mesured in bars... Of course, turbo engines will have a higher BMEP. 1 bar = 14.5psi. In 2006, according to this website, an F1 car had a BMEP of about 14.63. A Nextel cup car, 15.1.

Rabbit, I don't know how you did your math, the equation is (kW x 1200) divided by (L x rpm). Therefore the Hayabusa engine gets a BMEP of 14.1. Very good for a N/A motor.

Of course the BMEP is not the whole story, but it allows us to compare engines, even though the power, number of cylinders and displacement are different.

[petrified.rabbit]

i thought about this last night as soon as i shut my computer off, since his measure is in bar he is comparing how the engine acts like a pump.. in which case the comparison is against "optimal" efficiency. (air compressors only operate at one rpm)..


so if you have a firefly, 3 cylinder .9l making 51 kw at 5500 rpm = 12.3 bar all those vtec boys will have metro envy..


plus these are all stock configurations..


just for fun, dahlback racings gti:

2.1l making 527.6 kW @ 5.780 r/min = 52.16 bar...

[petrified.rabbit]

thats what ive been doing.. (kwx1200)/(lxrpm)

i dont know how i screwed up that first one..

[Shibbs]

You know why I like the Ecotec motor more? Usable torque curves. Actually, I could almost say "torque LINE"

[wylee]

You know why I like the Ecotec motor more? Usable torque curves. Actually, I could almost say "torque LINE"

I dunno, that dyno chart starts @ 3500rpm...pretty high for "normal" driving. I find the torque below 3000rpm pitiful in my 2010. I'd give up a few HP at 5000+rpm for better low-end torque, but I'm sure I'm in the minority; peak numbers sell more cars than a wider torque curve.

[petrified.rabbit]

i'm usually over 3k rpm when i drive.. its all about choosing your gearing than racing. unless your one of the hyper-milers, it keeps the car responsive.