Engine Talk

[eVo-XTreme]

Hmmm... does an engine with a maximum power of 118/5,500 (ps/rpm) faster compared to an engine with 118/6,200 (ps/rpm)??? San ba nalalaman kung ang engine ng isang car eh malakas or mabilis? Sa displacement lang ba ng engine? ^_^

[speedyfix]

power at a lower rpm will make the car feel faster because you will feel the acceleration earlier.

[eVo-XTreme]

i thought no one would ever try to reply on this post.. hehe... so speedefix, will a car with a lower rpm also affect its top speed?

[yebo]

alam mo medyo complicated yan question mo. are you asking same car with different engine and tranny installed or completely different cars?

kung same car with different engine and tranny, then they MAY have the same top speed since the weight, coefficient of drag, frontal area, tyre size and type, etc etc etc are the same. it will finally rest on the final drive ratio from engine-tranny-differential. IF the final drive ratio are the same then the higher speed engine MIGHT deliver a higher top speed ON THE ASSUMPTION that the power delivered is enough to propel the car to that higher top speed. but most of the time the higher revving engine will be mated to a higher drive ratio tranny and differential so the top speeds will not differ much.

if you're comparing different cars then there are as many possible answers to your question as there are car models. you can't possibly compare apples and peaches.

[eVo-XTreme]

Originally posted by yebo
alam mo medyo complicated yan question mo. are you asking same car with different engine and tranny installed or completely different cars?

kung same car with different engine and tranny, then they MAY have the same top speed since the weight, coefficient of drag, frontal area, tyre size and type, etc etc etc are the same. it will finally rest on the final drive ratio from engine-tranny-differential. IF the final drive ratio are the same then the higher speed engine MIGHT deliver a higher top speed ON THE ASSUMPTION that the power delivered is enough to propel the car to that higher top speed. but most of the time the higher revving engine will be mated to a higher drive ratio tranny and differential so the top speeds will not differ much.

if you're comparing different cars then there are as many possible answers to your question as there are car models. you can't possibly compare apples and peaches.

ganun po ba? hehe... sorry! medyo curious kasi ako when it comes to cars lalo na with their engines... i also like comparing different kinds of vehicles with one another... kaya siguro napakcomplicated na ng question ko... hahahaha! im just just a kid you know? LOLS! pero kung sa bagay, there are pros and cons on different engines.. am i right? anyway, bout the 2 engines im comparing with... 2 different cars ang tinutukoy ko... ^_^

[mazdamazda]

to better gauge the performance of the engine that you are referring to, maybe you can take a look at the following:

- hp / ps output over the whole rpm band. or a dyno chart.
- performance for 0-60kph, 60-100kph, 0-100kph, 100kph-160kph, 0-160kph.

[pepengtom]

anong kotse ba ung kinocompare mo?

[ebbfolls]

Power is power, regardless of where in the rpm band it is found. Given 2 engines of the same power output, and both of these engines are fitted into very similar cars, mated to transmissions that are geared appropriately so that the same amount of roadwheel horsepower(rwhp) is made at the driving wheels, then both cars will have the same performance, though they may sound different by reason of the 2 different torque and hp curves.

However, theoretically, the engine with the higher rpm will have less frictional losses through the driveline by reason of the lower gears it will be using, thus giving it a negligible power advantage over the other engine.

[speedyfix]

i think nagkakagulo and nagiging masyado complicated na itong thread.

basically you just need to know that if 2 engines make the same power at dfferent points, all other things being equal, then the one making it at lower rpm's will feel faster.

however, since in the real world not all things are equal and you have to take into consideration things like the weight of the car, gearing of the tranny, weight of the flywheel (how fast it revs), and a whole bunch of other stuff including torque and where that peaks, it will be hard to do a comparo by just looking at the stats.

i think they call this bench racing wherein you just look at the numbers and extrapolate data from there (i.e. if i do this to the engine, ito yung magiging gains ko and so on). well, using this is not the best means to see whether a car is faster than another or not kaya testing and a heads up comparison is always key

whew!

[ebbfolls]

To the threadstarter who wanted to know kung pano malalaman ang lakas ng engine-- sa POWER rating po yun. Power is the ability to produce work(or results) per a given amount of time.

Horsepower(HP) = Torque X RPM/5252



Mr. Speedyfix,

You are right about your argument about the variables being virtually impossible to control, as to make a valid test between 2 different engines. But in theory, 2 engines making the same amount of power, regardless of what rpm they make their max power, are equally POWERFUL one to the other. Now, say a Toyota 2.0 liter is making 160 HP at 5000 rpm and a Honda 2.0 liter is making 160 HP at 7000 rpm. Both engines are of the same power, thus both are capable of doing the SAME AMOUNT OF WORK, at the SAME AMOUNT OF TIME. (But because their max power figures occur at different engine speeds, both have different TORQUE figures at the rpm where they both make their maximum power. But torque only tells about how strong an engine is without regard for time, while HP tells about how fast an engine can produce results.)

So, if both engines are made to power cars of the same weight, both cars will have the same acceleration times and top speeds, because they are being propelled forward by engines that have the same amount of capacity to do work.

However, since you mentioned flywheel weight, then that would also have to be put nto consideration if we are to talk about acceleration. The engine with the less rotational mass(moment of inertia), will of course be able to accelerate quicker (By the way, a lighter flywheel only means the engine will be able to accelerate quicker to a certain rpm, but max rpm is still a function of the torque band, regardless of flywheel weight). However, top speed still be the same because both engines will only be able to accelerate to the point at which they can no longer produce more power to push the car against the resistance of the air and other frictional forces.

[eVo-XTreme]

*sighs*... super wala na akong maintindihan dito... wehehehe! pero like what ebbfolls said "Now, say a Toyota 2.0 liter is making 160 HP at 5000 rpm and a Honda 2.0 liter is making 160 HP at 7000 rpm. Both engines are of the same power, thus both are capable of doing the SAME AMOUNT OF WORK, at the SAME AMOUNT OF TIME. (But because their max power figures occur at different engine speeds, both have different TORQUE figures at the rpm where they both make their maximum power. But torque only tells about how strong an engine is without regard for time, while HP tells about how fast an engine can produce results.)"... kahit magkaiba ng max RPM ang isang car kahit pareho ng displacement, both are capable of doing the same amount of work at the amount of time... and the weight of the car also affects acceleration times and top speeds.... hehehe! now im quite enlightened with that answer... Thanks PEEPS! And just as i said... im just TOO curious about car engines... as in tagal ng umiikot sa utak ko ang question na yan... ^_^

[yebo]

Originally posted by eVo-XTreme
[B... as in tagal ng umiikot sa utak ko ang question na yan... ^_^ [/B]

wag mo masyado paikutin pards, dami na nadale dyan hehehe!

[kinyo]

the power vs. rpm curve of an engine looks like an inverted parabola, while load curve (required hp vs car speed) would look like the right-half of an upright parabola ... if the two curves were drawn on the same graph (with appropriate conversion of rpm into speed at highest gear), the top speed would then be the intersection of the two curves

if we now assume that the two engines' power curves have similar shape except that one's peak power occurs earliear than the other, then it can be concluded that the engine with higher rated rpm will have higher top speed ... but it can be expected that the difference in top speed would be minimal because the load curve is typically very steep at high speed

[eVo-XTreme]

Originally posted by yebo
wag mo masyado paikutin pards, dami na nadale dyan hehehe!

hahaha.... malapit na... LOLZ! ^_^

Mr. kinyo... thanks for the graph!

[ebbfolls]

Interesting concept.

What happens if the load curve is moved to the left so that it intersects both power curves before their power peaks?

[ian_rex]

in a sense yes. depende kung ano bang hanap mo. if you want acceleration, mas mabilis yung 118hp *5,500rpm but beyond that wala na kumpara sa 118hp *6200 rpm. pero sa top speed, i go for the latter kasi mas malaki ang power band nya kesa dun sa nauna, considering na pareho lang sila ng output.

based on my experience, mas maganda yung nasa top end ng rpm yung power para malaki ang power band. lalo na yung sa torque. sa diesels ganyan ang lakas ng torque kaya lang sa low to mid rpm lumalabas kaya pag sa long drive na high speed mabagal sya. pero matipid.

[kinyo]

Originally posted by ebbfolls
Interesting concept.

What happens if the load curve is moved to the left so that it intersects both power curves before their power peaks?

Then Engine A will end up with higher top speed.

The question now would be ... "What happens to the remaining HP that both engines are capable of generating? Won't that extra HP supposed to increase the speed further?"

The answer is "No". At any speed before the top speed is reached, the difference between the load curve and an engine's power curve is the HP being used to accelerate the car. When top speed is reached, acceleration becomes zero, speed becomes constant. Beyond the top speed, the load curve becomes higher than an engine's power curve, which means that any increment in speed requires more HP than the engine could produce for the same speed increment. Hence, the extra HP from an engine cannot be used to further accelerate the car and pressing on the pedal further would only result to waste.

Moving the load curve to the left might be akin to changing the aerodynamics of the car where wind drag is increased. If we deploy a parachute on the rear of the car, its top speed would be severely limited and pushing the engine further would only waste the tires while speed remains constant.

[ebbfolls]

Originally posted by kinyo
Moving the load curve to the left might be akin to changing the aerodynamics of the car where wind drag is increased. If we deploy a parachute on the rear of the car, its top speed would be severely limited and pushing the engine further would only waste the tires while speed remains constant.

It would also be akin to using a small displacement, high rpm engine to power a vehicle with a relatively large frontal area.

Using the appropriate reduction gearset would also bring the engines power peak closer to the load curve slightly improving the vehicle's top speed, dont you think? But I think this only applies to situations where the load curve intersects the power curve before max power peak.

[ebbfolls]

I've plotted a graph similar to the one you posted. This is the power output of the same engine measured at the output end of 2 reduction gear ratios. As can be seen, both power curves have the same HP levels as power does not change regardless torque reduction or multiplication, assuming negligible friction.

It's interesting to note how the difference between the HP requirement and the HP output of the engine is much greater with the lower gear from Point A to Point B. Points C and D, on the other hand, point to the increase of the top speed afforded by the taller gear.

The 'area under the curve' should probably equate to energy expenditure.

You are welcome to correct any misconceptions.


(Sorry, that didnt work. How do you paste a picture here anyway?)

[kinyo]

Use the "New Reply" button. You will then see the "Attach file" feature.