Highreeving engine - at what rpm?

[Deleted member 182]

it's just nor right to call a grocery getter a High-revving car. know what I mean?

What about BMW M5 touring?

 

[NarutoRamen]

What about BMW M5 touring?

V10 engine isn't really a grocery getter car now is it?

I wasn't talking about cars in general btw...I was referring to my honda because hondas and toyotas are usually called grocery getters in the US.

 

[chonkoa]

Ooo vedjo01

I don't think that by increasing piston speeds you will necessarily lose torque; torque is a product of displacement and not piston speed. Increasing piston speed will produce more power since this is a product of either:

- Stroke length
- RPM

Torque by itself is not RPM dependent, however engine torque is because of the cycle speed.


Engine Torque=P*A*R*Z
P= effective piston Pressure
A= Piston cross sectional area
Z= Number of pistons/Cylinders
R= Radius arm of the piston.

Because we are dealing with a dynamic state, the effective piston pressure is engine speed dependent.
The effective pressure is dependent on the volume of fuel+air mixture:heavier fuel will give more pressure, so will more air in the cylinder
The effective piston pressure is maximum at zero speed and exponentially decay with increasing engine speed. This decay is due to not having enough time for air to fill combustion chamber(turbo engines addresses this problem)

So then, why does torque increase at then decrease? The reason for this are the following:.

1. At zero speed, only half of the cylinders are generating torque, and since this is a cycle, you have to average for the entire cycle.
2. As you increase the speed, more cylinders/unit time contributes to the engine torque, however the torque generated per cylinder decreases exponentially
3. There reaches a speed where the torque generated per cylinder/stroke is so low that the overall engine torque starts to decrease.

 

[martinbo]

Aw gee, you know how long I've been waiting for that post! Hey chonkoa - good work dude! Nice and concise explanation that makes perfect sense.

Now that's the kind of posting we need in the tech talk forum...

By radius arm of the piston I gather this refers to the stroke length?

 

[Imhotep Evil]

Torque by itself is not RPM dependent, however engine torque is because of the cycle speed.


Engine Torque=P*A*R*Z
P= effective piston Pressure
A= Piston cross sectional area
Z= Number of pistons/Cylinders
R= Radius arm of the piston.

Because we are dealing with a dynamic state, the effective piston pressure is engine speed dependent.
The effective pressure is dependent on the volume of fuel+air mixture:heavier fuel will give more pressure, so will more air in the cylinder
The effective piston pressure is maximum at zero speed and exponentially decay with increasing engine speed. This decay is due to not having enough time for air to fill combustion chamber(turbo engines addresses this problem)

So then, why does torque increase at then decrease? The reason for this are the following:.

1. At zero speed, only half of the cylinders are generating torque, and since this is a cycle, you have to average for the entire cycle.
2. As you increase the speed, more cylinders/unit time contributes to the engine torque, however the torque generated per cylinder decreases exponentially
3. There reaches a speed where the torque generated per cylinder/stroke is so low that the overall engine torque starts to decrease.

Thanks.

So there actually is a way to keep up (some) torque as the rpms/speed goes up. In the old F1 V10s they had variable intake systems, variable exhaust systems and variable valve control systems, and (still have) control over air-fuel mixture. Constant adjustments to this from the steering wheel buttons/switches meant a better torque curve.

 

[Zafiro]

the new Skyline is getting Turbo and redline at 7600 rpm. Pretty high for a Turbo car.

 

[NarutoRamen]

the new Skyline is getting Turbo and redline at 7600 rpm. Pretty high for a Turbo car.

The japanese have always had Hi-revving Turbo cars. I don't know how, but they always manage.

 

[martinbo]

The japanese have always had Hi-revving Turbo cars. I don't know how, but they always manage.

I've heard of JDM STI Spec C's running to 7500 rpm. That's very high for a turbo'd mill.

 

[NarutoRamen]

I've heard of JDM STI Spec C's running to 7500 rpm. That's very high for a turbo'd mill.

hmm, I'll have to look into that. My favourite is the (GC8) Impreza STI Type-R...just love that thing. Didn't that also have a pretty high redline?

 

[speed_kills]

I think it is 7250 for the Impreza. I remember seeing in the net a Evo revving to 8000, on the redline of course...

 

[NarutoRamen]

I think it is 7250 for the Impreza. I remember seeing in the net a Evo revving to 8000, on the redline of course...

Thanks for confirming it for me...7250 is still pretty damn high, especially considering it's 2.2L motor pumping out 290bhp/263lb-tq...that's just a little bit over 130bhp/liter, out of a 4banger....now that is badass.

 

[vedjo01]

Thx,Naruto..this is "THE IMPREZA" IMO..This new version just doesn´t have appeal of old beast(I´m quite szre it´s beast under the skin ).Japan cars never stop to amaze me..just how do they do it ?

 

[NarutoRamen]

Thx,Naruto..this is "THE IMPREZA" IMO..This new version just doesn´t have appeal of old beast(I´m quite szre it´s beast under the skin ).Japan cars never stop to amaze me..just how do they do it ?

yep, I'm with you on this one. That right there is my favorite impreza, the new ones are good too but I just love the look of the old one. The old had a brutish look with a little bit of style. The new ones are just straight up brutes. But the way the new hatch is turning out to be, I've gained some love back.

Now this question is for the technical guys. I've heard that the more cylinders you have the harder it is to get a higher redline? is that true or is the the opposite? I'm confused on this one. Martinbo, chonkoa, care to help?

 

[Sunny]

yep, I'm with you on this one. That right there is my favorite impreza, the new ones are good too but I just love the look of the old one. The old had a brutish look with a little bit of style. The new ones are just straight up brutes. But the way the new hatch is turning out to be, I've gained some love back.

Now this question is for the technical guys. I've heard that the more cylinders you have the harder it is to get a higher redline? is that true or is the the opposite? I'm confused on this one. Martinbo, chonkoa, care to help?

For a given displacement, it is the opposite. So a 5L V10 can have a higher redline than a 5L V8 (everything else being equal). The biggest impediment to a higher redline is the weight of the moving parts - pistons, connecting rods etc - the heavier they are, the slower they will accelerate and lower the redline (also the stresses go up with weight - making it even worse). And for a given displacement, the more the number of cylinders, the lighter the individual pistons/cylinders and hence higher they can rev.

So the obvious follow on question is, why not make every engine a V20 or V50 with really small cylinders/pistons and rev it to the heavens - but unfortunately as you increase the number of cylinders, the engine as a whole gets less efficient cause of higher parasitic looses: for a given displacement, the more the number of cylinders, the higher the contact area between the cylinders and pistons increasing friction. So it is a balancing act between these two factors. Also more the number of cylinders, higher the manufacturing and maintenance cost. So like most other things in life, it is a balancing act.

Of course, I am simplifying things here in my very limited knowledge.

As for the 1st gen WRX, my favorite too, fell in love with it playing the first Gran Turismo along with the Skyline GTR.

 

[chonkoa]

yep, I'm with you on this one. That right there is my favorite impreza, the new ones are good too but I just love the look of the old one. The old had a brutish look with a little bit of style. The new ones are just straight up brutes. But the way the new hatch is turning out to be, I've gained some love back.

Now this question is for the technical guys. I've heard that the more cylinders you have the harder it is to get a higher redline? is that true or is the the opposite? I'm confused on this one. Martinbo, chonkoa, care to help?

In addition to what Sunny just said which in my limited opinion is 100% spot on.

I believe the most important factor with rev limit is the combustion pressure in a cylinder.
Some of the factors that affect combustion pressure are:
1. The type of fuel mixture for a given displacement. A good example is between Petrol and Diesel. A diesel combustion generates more pressure than Petrol due to the heavier Carbon and Hydrogen composition. The higher combustion pressure is due to more exhaust gases that are generated per stroke. There is a limited space(outlet valves) for the gas to escape which limits how fast the engine can make the next stroke, thus a lower redline than petrol engine. The same can be said of Petrol versus Natural gas engine, the natural gas will rev faster than the petrol engine because of the lower combustion pressure. It is important to note that this combustion pressure is what gives the engine its torque.
2. For the given fuel mixture, the displacement affects the redline like Sunny pointed out. If you increase the displacement by increasing the stroke length instead of the bore diameter, the piston will have to travel a longer distance per stroke, thus reducing the redline. One way of dealing with this is to increase the bore diameter rather than the stroke length. Either way, increasing the displacement will increase the combustion pressure of the cylinder thus creating the same limiting effect on the engine speed.

 

[Sunny]

In addition to what Sunny just said which in my limited opinion is 100% spot on.

I believe the most important factor with rev limit is the combustion pressure in a cylinder.
Some of the factors that affect combustion pressure are:
1. The type of fuel mixture for a given displacement. A good example is between Petrol and Diesel. A diesel combustion generates more pressure than Petrol due to the heavier Carbon and Hydrogen composition. The higher combustion pressure is due to more exhaust gases that are generated per stroke. There is a limited space(outlet valves) for the gas to escape which limits how fast the engine can make the next stroke, thus a lower redline than petrol engine. The same can be said of Petrol versus Natural gas engine, the natural gas will rev faster than the petrol engine because of the lower combustion pressure. It is important to note that this combustion pressure is what gives the engine its torque.
2. For the given fuel mixture, the displacement affects the redline like Sunny pointed out. If you increase the displacement by increasing the stroke length instead of the bore diameter, the piston will have to travel a longer distance per stroke, thus reducing the redline. One way of dealing with this is to increase the bore diameter rather than the stroke length. Either way, increasing the displacement will increase the combustion pressure of the cylinder thus creating the same limiting effect on the engine speed.

Thanks, I knew combustion played a part too, was just not sure how.

Going back to turbo engines and red line, I can't think of a reason why turbo engines have inherently a lower redline. I think the reason why turbo engines usually have a lower redline is cause with a turbo engine it is easier (and cheaper) to increase output by just increasing the turbo pressure rather than increasing the redline - just manufacturers resort to just increasing the turbo pressure than increasing the redline.

One reason I can think of is that the fans driving the turbos are not efficient across a wide RPM range. But the new variable fans in the 997 turbo addresses that.

 

[NarutoRamen]

^^Thanks guys for the info. Now I have some reading to do. I'm gonna go ahead and read a few technical definitions so I can expand my car knowledge. It's good to have smart technical guys on this board.