How an Engine works
11-05-2008, 08:06 PM
#1
Registered User
Thread Starter
How an Engine works
I wanted to post this a while back, but forgotten where i'd found it.
Stumbled upon it again
The bloke 'Liveforphysics' has given us a truly wonderful post that goes on to explain the Birds and the Bees of engines in a rather excellent way.
Here is his post:
[QUOTE=Liveforphysics]
Now to get serious.
Transient response is not the rate at which an engine free revs, nor is it related to polar moment. Transient response is the engines ability to carry a load as RPM changes at real world acceleration rates.
This has only a small part to do with polar moment of the crankshaft, and is largely unrelated to the recipro mass. Often transient response is increased by the addition of higher mass dampeners like fluid-dampers, even though the free rev rate is decreased. This becomes a complex problem of energy robbing harmonic crank deflection harmonics that I won't bother talking about, as the lowerend has very little to do with transient response aside from the swept volume it demands, which is hugely impactful.
Ok, so what is transient response you ask then? Well, lets break down how a car accelerates. In a car without a CVT or a slipping clutch or converter, to accelerate we must increase the RPM of the engine while under load.
How does a header work? How does an intake manifold work? How does the cam profile work with the header and intake runner?
Answer:
They all feed from the previous pulse. When operating in the range they are tuned for, the previous pulses (intake and exhaust) provide them with some constructive advantage towards the next stage in aspiration.
For example, when a proper header is in the correct RPM range and throttle position, it is able to use the previous firing cylinders exhaust pulse velocity to drop the pressure in the exhaust port to begin the exhaust exit process effectively quite a bit before the piston begins to rise. Similar effects happen in the intake runner/port/bowl area. The previous pulse sets the stage in a constructive mannor to help the next pulse perform better. How much better? A HUGE amount better.
For me, transient response is more about two things. The firs is how quickly an engine is able to make power again after the very power disruptive process of jerking the RPMs down on a shift. This has absolutely nothing to do with mass of recipro and/or polar moment of the rotational assembly. This is entirely about how many cycles it requires before those pulses can constructively help eachother again after a shift.
As engines become more tuned, they use these sorts of tuning effects more and more, which is why it becomes so impactful on engines like the F2XC and things like street bike engines. For example, on my GSXR1000, everytime I grab a new gear, the engine always makes a small "lul" in the induction noise often over a 1/8th of a second while the engine deals with getting pulse timing back to become constructive again. For this period, a notable drop in power is observed, followed by the engine pulling at full stregnth again.
In my lightly tuned F20C powered 69 datsun 510, this effect is also easily noticed, yet in my stock S2000 I can hardly feel this due to the stock engine being designed around a quick recovery.
Now, the second part of transient response. You will note that an engines ideal spark timing is never the timing that creates peak torque at dyno sweep rates, nor is it the threshold of knock on the dyno. It's always a degree or two past that point when really accelerating because the cylinder fill is never as complete during real life sweep rates. Why is the fill not as good? Because the previous pulse is not setting up the next cylinder properly due to the timing being all wrong due to the rapid rate of change in RPM. If you are going to make a top speed pull, by all means tune the car on the dyno (not that manifold pressures and temps when at speed are ever like dyno conditions...), and stick with whatever pops the biggest numbers on the dyno. If you happen to be looking for quick acceleration, the dyno numbers begin to play a much lesser roll over the engine designed to accelerate under a load as rapidly as possible.
John (hitechex) can tell you that in formula atlantic (he makes most of the good teams headers for FA), he has produced headers that caused lower HP and torque numbers at all measured RPM points on the dyno, yet the cars had there fastest lap times. The drivers reported that the cars felt more powerful as they pulled the gears faster. The engine had less dyno measured torque at all RPM points.... So, why did it work better? He set the header up (or got lucky
Stumbled upon it again
The bloke 'Liveforphysics' has given us a truly wonderful post that goes on to explain the Birds and the Bees of engines in a rather excellent way.
Here is his post:
[QUOTE=Liveforphysics]
Now to get serious.
Transient response is not the rate at which an engine free revs, nor is it related to polar moment. Transient response is the engines ability to carry a load as RPM changes at real world acceleration rates.
This has only a small part to do with polar moment of the crankshaft, and is largely unrelated to the recipro mass. Often transient response is increased by the addition of higher mass dampeners like fluid-dampers, even though the free rev rate is decreased. This becomes a complex problem of energy robbing harmonic crank deflection harmonics that I won't bother talking about, as the lowerend has very little to do with transient response aside from the swept volume it demands, which is hugely impactful.
Ok, so what is transient response you ask then? Well, lets break down how a car accelerates. In a car without a CVT or a slipping clutch or converter, to accelerate we must increase the RPM of the engine while under load.
How does a header work? How does an intake manifold work? How does the cam profile work with the header and intake runner?
Answer:
They all feed from the previous pulse. When operating in the range they are tuned for, the previous pulses (intake and exhaust) provide them with some constructive advantage towards the next stage in aspiration.
For example, when a proper header is in the correct RPM range and throttle position, it is able to use the previous firing cylinders exhaust pulse velocity to drop the pressure in the exhaust port to begin the exhaust exit process effectively quite a bit before the piston begins to rise. Similar effects happen in the intake runner/port/bowl area. The previous pulse sets the stage in a constructive mannor to help the next pulse perform better. How much better? A HUGE amount better.
For me, transient response is more about two things. The firs is how quickly an engine is able to make power again after the very power disruptive process of jerking the RPMs down on a shift. This has absolutely nothing to do with mass of recipro and/or polar moment of the rotational assembly. This is entirely about how many cycles it requires before those pulses can constructively help eachother again after a shift.
As engines become more tuned, they use these sorts of tuning effects more and more, which is why it becomes so impactful on engines like the F2XC and things like street bike engines. For example, on my GSXR1000, everytime I grab a new gear, the engine always makes a small "lul" in the induction noise often over a 1/8th of a second while the engine deals with getting pulse timing back to become constructive again. For this period, a notable drop in power is observed, followed by the engine pulling at full stregnth again.
In my lightly tuned F20C powered 69 datsun 510, this effect is also easily noticed, yet in my stock S2000 I can hardly feel this due to the stock engine being designed around a quick recovery.
Now, the second part of transient response. You will note that an engines ideal spark timing is never the timing that creates peak torque at dyno sweep rates, nor is it the threshold of knock on the dyno. It's always a degree or two past that point when really accelerating because the cylinder fill is never as complete during real life sweep rates. Why is the fill not as good? Because the previous pulse is not setting up the next cylinder properly due to the timing being all wrong due to the rapid rate of change in RPM. If you are going to make a top speed pull, by all means tune the car on the dyno (not that manifold pressures and temps when at speed are ever like dyno conditions...), and stick with whatever pops the biggest numbers on the dyno. If you happen to be looking for quick acceleration, the dyno numbers begin to play a much lesser roll over the engine designed to accelerate under a load as rapidly as possible.
John (hitechex) can tell you that in formula atlantic (he makes most of the good teams headers for FA), he has produced headers that caused lower HP and torque numbers at all measured RPM points on the dyno, yet the cars had there fastest lap times. The drivers reported that the cars felt more powerful as they pulled the gears faster. The engine had less dyno measured torque at all RPM points.... So, why did it work better? He set the header up (or got lucky
....
11-06-2008, 01:09 AM
#5
Registered User
Join Date: Jan 2008
Location: West Sussex
Posts: 204
Likes: 0
Received 0 Likes
on
0 Posts
At no time, ever, have I read a description of how the engine (or even VTEC) works that explains it in words of sufficient simplicity for engineering dummies like me.
I've never understood it.
I've never understood it.
11-06-2008, 02:08 AM
#6
Registered User
Originally Posted by Papillon,Nov 6 2008, 02:09 AM
At no time, ever, have I read a description of how the engine (or even VTEC) works that explains it in words of sufficient simplicity for engineering dummies like me.
I've never understood it.
I've never understood it.
It's really that simple*
*To get one running... Once you get beyond running and into tuning it all becomes slightly more complicated.
11-06-2008, 02:47 AM
#7
Registered User
Join Date: Jan 2008
Location: West Sussex
Posts: 204
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by GaryB,Nov 6 2008, 11:08 AM
Suck, squeeze, bang, blow. Fuel, air and a spark (at the right time) and you're away.
It's really that simple*
*To get one running... Once you get beyond running and into tuning it all becomes slightly more complicated.
It's really that simple*
*To get one running... Once you get beyond running and into tuning it all becomes slightly more complicated.
(Awaits abuse)
Thread
Thread Starter
Forum
Replies
Last Post
DaveB
UK & Ireland S2000 Community
10
03-06-2006 04:05 AM
