How high HP/Torque can NA engines get to?

[454Casull]

Depends on volumetric efficiency throughout the range of RPMs, maxiumum RPM, displacement, bore/stroke, combustion chamber shape, A/F ratio, fuel atomization, material selection (mainly to increase the efficiency of conversion from heat to pressure), total friction, etc. etc. etc.

[454Casull]

A question for all you people: what do you think efficiency means?

[454Casull]

Originally posted by flylwsi
i am not in any way saying that the numbers have to be exactly equal by any means...

and fyi, the s2k makes 175 tq at about 7500, not 151...

diesel isn't something i'm even going to look at, b/c they're tq monsters, not hp based vehicles...

The 2003 S2000 makes 153 lb-ft at 7500rpm.

And that last comment is flawed. Efficiency is not about the ratio between power and torque.

[SaabJohan]

One way to measure how "highly tuned" (has nothing to do with thermal efficiency) an engine is, is by measure the brake mean effective pressure (bmep). If an engine has a high bmep value it usually gives a high power output per displacement if the engine can use higher rpms. High rpm engines usually tend to have lower bmep than low rpm engines.

Another way to measure this is by using power/displacement, often hp/litre. Here it's an advantage to use high rpms. In racing it's important to have a high power/displacement ratio, not only because many classas are displacement limited but also because this allows a light and compact engine. Also, if the power is created by high engine speed instead of large volume the torque will be lower and that is actually a good thing. The torque is lower but the power stays the same, this means that the car will have the same performance as a car with the same power but a higher torque; the difference is that a lighter drivetrain can be used since the torque is reduced (power or torque at the wheels accelerates the car, engine torque destroy transmissions).

Some people tend to claim that power/displacement is misleading since larger engines can't rev that high. Yes, larger engines can't rev as high as smaller engines, but engines with few cylinders or long stroke have the same disadvantage. This is no stranger than a larger engine has a lower thermal efficiency as well as a high rpm engine has a lower efficieny than a low revving one. Just basic physics that every engine designer should know. If we want a small, light engine with a high power output the choice is to use a small high revving engine with a short stroke and many cylinders. On the other hand, if we are going to build an engine for maximum thermal efficiency we shall chose a low revving engine with a long stroke and few cylinders and with the smallest displacement possible.

Also note that highly tuned engines have low torque/power at low rpms, this is simply because their camshafts are design to work well att high rpms with higher flow speeds.

If we have two engines both producing the same torque, one of them is larger than the other but also more low revving. The smaller and high revving engine will produce more power since they produce the same torque. If we put both engines in cars the smaller engine will outperform the larger one. If we further increase the rpm but also lower the torque we can still achieve the same power output as before and the larger engine will again be outperformed.

An optimum torque curve for an NA engine is flat. If the torque curve is flat the power increase with rpm and never drops. Engineers are today working to make the torque curve as flat as possible. But to make a flat curve with maxmum power output is difficult.

[454Casull]

Originally posted by SaabJohan
One way to measure how "highly tuned" (has nothing to do with thermal efficiency) an engine is, is by measure the brake mean effective pressure (bmep). If an engine has a high bmep value it usually gives a high power output per displacement if the engine can use higher rpms. High rpm engines usually tend to have lower bmep than low rpm engines.

Another way to measure this is by using power/displacement, often hp/litre. Here it's an advantage to use high rpms. In racing it's important to have a high power/displacement ratio, not only because many classas are displacement limited but also because this allows a light and compact engine. Also, if the power is created by high engine speed instead of large volume the torque will be lower and that is actually a good thing. The torque is lower but the power stays the same, this means that the car will have the same performance as a car with the same power but a higher torque; the difference is that a lighter drivetrain can be used since the torque is reduced (power or torque at the wheels accelerates the car, engine torque destroy transmissions).

Some people tend to claim that power/displacement is misleading since larger engines can't rev that high. Yes, larger engines can't rev as high as smaller engines, but engines with few cylinders or long stroke have the same disadvantage. This is no stranger than a larger engine has a lower thermal efficiency as well as a high rpm engine has a lower efficieny than a low revving one. Just basic physics that every engine designer should know. If we want a small, light engine with a high power output the choice is to use a small high revving engine with a short stroke and many cylinders. On the other hand, if we are going to build an engine for maximum thermal efficiency we shall chose a low revving engine with a long stroke and few cylinders and with the smallest displacement possible.

Also note that highly tuned engines have low torque/power at low rpms, this is simply because their camshafts are design to work well att high rpms with higher flow speeds.

If we have two engines both producing the same torque, one of them is larger than the other but also more low revving. The smaller and high revving engine will produce more power since they produce the same torque. If we put both engines in cars the smaller engine will outperform the larger one. If we further increase the rpm but also lower the torque we can still achieve the same power output as before and the larger engine will again be outperformed.

An optimum torque curve for an NA engine is flat. If the torque curve is flat the power increase with rpm and never drops. Engineers are today working to make the torque curve as flat as possible. But to make a flat curve with maxmum power output is difficult.

Good post, but there are two things I'm concerned about. 1: Can you explain how a larger engine is less thermally efficient? And 2: variable-timing cams can flatten the torque curve, especially with the fully-variable kind.

[FYRHWK1]

less thermally efficient would be simply that theres more area for heat to transfer to, and more liquids to take heat away from the combustion chamber. With a larger engine variable valve timing isn't quite as missed, it can have a camshaft profile capable of pulling until redline but still produce enough torque down low to move the car, in fact most do this ion purpose to prolong the life of the engine since high cyl pressures at low rpms are more damaging.

[ivymike1031]

let's make sure we all mean the same thing by thermal efficiency, before we get into arguing about different quantities.

I propose that the following definition of thermal efficiency be used for this discussion:
Eff.thermal = (Work output per cycle, at flywheel) / (Heat added per cycle),
where (heat output per cycle) is the heat added after any inefficiencies in the combustion process are considered (Q.in = Q.HV * m.fuel * Eff.combustion)

It is not apparent to me that large-displacement engines will necessarily be less thermally efficient than small-displacement engines. A slower-turning, larger-displacement engine can have considerably lower hydrodynamic friction losses than a small, faster rev'ing engine of the same power output.

[SaabJohan]

Smaller displacement mean lower heatlosses. But if higher engine speed is used the frictionlosses will increase. So if we are going to build a thermal efficient engine we should have a small displacement and a low engine speed.

The most common variable cams are very similar to adjustable cam gears just like this one:
http://www.hksusa.com/images_products/1250.jpg
The difference is that they are controlled by the engine ECU and oilpressure. The simplest ones have one point where the ECU opens a valve that lets the oilpressure to change the cam timing.
If someone have tested an adjustable cam gear like the HKS on an engine in a bench, you will soon find out by changing the timing the low end power and top end power will depend on the setting on the gear.
So what the engine manufacturers do is that they choose two settings; one for low end and one for high end power. Where the power curves match, that's the adjusting point when the ECU will open the valve that lets the oilpressure change the timing.

[454Casull]

Originally posted by SaabJohan
Smaller displacement mean lower heatlosses. But if higher engine speed is used the frictionlosses will increase. So if we are going to build a thermal efficient engine we should have a small displacement and a low engine speed.

The most common variable cams are very similar to adjustable cam gears just like this one:
http://www.hksusa.com/images_products/1250.jpg
The difference is that they are controlled by the engine ECU and oilpressure. The simplest ones have one point where the ECU opens a valve that lets the oilpressure to change the cam timing.
If someone have tested an adjustable cam gear like the HKS on an engine in a bench, you will soon find out by changing the timing the low end power and top end power will depend on the setting on the gear.
So what the engine manufacturers do is that they choose two settings; one for low end and one for high end power. Where the power curves match, that's the adjusting point when the ECU will open the valve that lets the oilpressure change the timing.

True, engines with smaller displacements lose less heat, but the ratio of heat loss:displacement is higher than a larger engine (assuming same bore/stroke ratio) because, IIRC, as a cylinder "grows" (again keeping bore/stroke ratio) its ratio of volume to surface area increases.

I think I just confused myself.

[ivymike1031]

454, it sounds like you're on the right track to me. The most efficient piston engines that I'm aware of (thermal) are extremely large displacement engines (vs automotive) that run very slowly (~100 rpm).

Typically one has power requirements determined by an application, and cannot simply opt for a small displacement, low speed engine. For a given power output, it seems to be generally true that a large-displacement, slow engine is more efficient than a small-displacement, high speed engine. (talking piston engines only)

Then again, arguing about the relative efficiency of gasoline-fueled engines is a bit like pissing into the wind while discussing whose feet are wetter - best to find another hobby.

[Steel]

soo... what would be the thermal effeciency of THIS?!
http://performanceunlimited.com/illu...ul-engine.html

[ivymike1031]

based on the link you provided, "over 50%."

[pod]

who the hell would need a 3 story engine

[454Casull]

Cruise ships.