Gas Efficiency

[GreyGoose006]

could someone please explain the graph that we have all been discussing.
i may understand the whole thing better if i knew what the graph meant.
:-]

[KiwiBacon]

Originally Posted by GreyGoose006

could someone please explain the graph that we have all been discussing.
i may understand the whole thing better if i knew what the graph meant.
:-]

Sure

Horizontal axis is engine speed (rpm)
Vertical axis is manifold pressure. Max vacuum at the bottom, full open throttle at the top.

The islands are BSFC (brake specific fuel consumption), they are marked with how many grams of fuel it takes to provide 1kw of engine output for 1 hours. Smaller numbers are more efficient. Lowest BSFC is less than 250g/kwH, highest BSFC is more than 550 g/kwH.

The thick blue line across the top is torque, units are Nm (Newton Metres) and the scale is on the right hand side. Max torque is approx 240Nm from 3000-4000rpm.

The lines which arc across the graph from top left, then down and across are constant power lines. The red one is 30kW, this engine maxes out at 120kW.

[GreyGoose006]

so if rpm and torque are directly related, why dont engines w/ lots of torque low down get better power (excluding desiels)
is it because of the fact that you have to close the throttle more?

[KiwiBacon]

Originally Posted by GreyGoose006

so if rpm and torque are directly related, why dont engines w/ lots of torque low down get better power (excluding desiels)
is it because of the fact that you have to close the throttle more?

I think it's due to engine design.
Most engines are either designed for good volumetric efficiency down low (which gives good low end torque and very good economy) or for good volumetric efficiency up high (which gives lots of power, great top end but little down low).

To bridge the gap between the two, manufacturers are using such things as variable length inlet manifolds and variable valve timing.

A very good example is toyotas FE engines compared to their GE engines.
The corolla I owned with the 5AFE was over 25 seconds to 100 km/h but was quite economical.
Simlar car with the 4AGE would be 1/3 to 1/2 of that time.

[Steel]

becasue power is just a mathematical toy, really.

HP = (Tq x RPM)/5252 The reason most motors with lots of torque down low (and this INLCUDES diesels, I don't understand why you would want to exclude them?) is that they usually will not spin all that fast, and are designed with this in mind. It may be in the case they they are big and heavy motors and/or have a very long stroke (such is the case with heavy diesels) or the intake is designed for velocity and not max flow i.e. the motor will choke if you try to spin it faster.

On the other end of the spectrum you have motors such as F1 engines that have terrible torque figures (something like 100 foot pounds, dont quote me) considering their displacement, but put out 700+ HP. Hoooow do they do it? They spin to 18,000+ rpm.

So the F1 car will beat the Mack truck to the finish line, but if i have 40 tons to tow, i think i'll pick the truck (not that the car sin't capable, it can do it fine, you'd just need a transmission with 100 speeds to get anywhere)

[GreyGoose006]

the reason i excluded diesels is that like you said, w/o a lot of work, most diesels only rev to like 4500 rpm or less.
...i dont know...

FINE diesels get to play too... if that makes you happy :-]

[KiwiBacon]

Originally Posted by GreyGoose006

the reason i excluded diesels is that like you said, w/o a lot of work, most diesels only rev to like 4500 rpm or less.
...i dont know...

FINE diesels get to play too... if that makes you happy :-]

Why would you even want to compare diesels to petrols in this context?

Bananas and apples.

[GreyGoose006]

well that was my original reason for saying "diesels excluded"

[Franko914]

HAHAHAHA!!! That's a pretty good way of explaining torque and horses. If it's all the same to you folks, I'll take the F1...

Anyhow, some bolt-ons do work on turbo-charged engines, but significantly more so for turbo-charged diesel engines, e.g., a $3,000 intercooler setup or a $300 water-injection setup; propane intake fumigators; NO2 kits; racing stripes...

The added horses/torque come at a price: added fuel consumption UNLESS you increase the engine's efficiency.

Because there is a finite amount of energy in a fixed amount of fuel (petrol or diesel), increasing an engine's efficiency is the only way to extract more work out of the fuel without increasing fuel consumption.

Gasoline engines have a fixed stoichiometric A:F ratio (~14:1). Diesels are not limited by such Fuel:Air ratio and can run anywhere from 10:1 to 150:1. Cold air intakes and free-breathing filters come into play in the realm of the higher A:F ratios.

Anyway, to add to the list of fuel-saving tips:

-- replace engine-driven cooling fan with thermostat-controlled electric fans
-- drive at the engine's max-torque RPM as much as possible
-- install larger diameter tires for mostly highway driving, or smaller diameter tires for mostly stop-n-go city driving (Yep! Higher pressure tires require less energy because it takes energy to continually squish tires as you drive along)
-- reduce load on alternator by turning off unneeded juice-burning accessories
-- consider re-sizing accessory pulleys depending on your needs

Good luck!