How to plot a power and torque graph for an engine?

[Sai Bhagavan]

TL;DR

Plot power and torque graph of an engine using peak power value at an RPM.

Hello all,
I have designed an intake manifold for boxer-6 naturally aspirated petrol engine with 1:1 bore to stroke ratio and has a bore dia of 117.89mm and a volume of 7.8l as part of my master's project. The engine was to produce 450 bhp @ 6500 rpm and I did set a rev limit @7500 rpm. The problem is I don't know at what rpm the engine produces it's maximum torque and how to find that. The amount of torque it produces at 6500 is 492.8Nm. Can I plot a graph between power and torque just by using the maximum power value and if how? Thanks in advance.

 

[Baluncore]

Welcome to PF.

Can I plot a graph between power and torque just by using the maximum power value and if how?

Power = torque * RPM.

The amount of torque it produces at 6500 is 492.8Nm.

6500 RPM = 6500*2*Pi/60 = 680.7 radian per second.
Power = 492.8Nm * 680.7 = 335438. watt = 335.438 kW = 449.8 HP

 

[Sai Bhagavan]

Hello thanks for the reply and the welcome. I wanted to know the maximum torque number and at which RPM it is produced.

 

[Baluncore]

I think that you will need more data. You should start by plotting torque against RPM.
Then compute the power from that and plot it on the same graph.

With the data you now have you can only plot one torque point = 492.8Nm at 6500 RPM.

To guess more points you must look at other plots for similar motors to see how the shape of the torque to RPM curve changes.

 

[Sai Bhagavan]

I have some doubdts though. Even if I find at other plots for a similar configuration engine, can we generalize the same trend for my project also?

 

[cjl]

No, you'll need the data from your engine. If you only know power at one RPM value, you only know torque at that same RPM. How did you obtain the power/torque figures for 6500RPM?

 

[Sai Bhagavan]

Well I have designed the intake on my own for the specified parameters. It is my own project. So am stuck with single values of power and torque each.

 

[anorlunda]

I wanted to know the maximum torque number and at which RPM it is produced.

So am stuck with single values of power and torque each.

Those two things don't go together.

 

[cjl]

Well I have designed the intake on my own for the specified parameters. It is my own project. So am stuck with single values of power and torque each.

How do you know the engine will make that power though? If this is just a hypothetical, sure, you can reasonably estimate based on similar engines.

Also, just as a side note, 7500RPM seems very high for a 7.8l six cylinder engine. Are you expecting it to be hugely oversquare?

 

[Sai Bhagavan]

Well it is a hypothetical engine. Exists only in my dreams. I just wanted to assume how that engine would be as most of the engines are not square so I just wanted to design a road going square engine as part of my project.

 

[cjl]

Ahh. In that case, you can assume any torque curve you want. In reality, it'll obviously depend on head design, valve area, cam profile, etc.

Also, a lot of road going engines are square, or close to it. Some modern efficiency-focused designs are going a bit more in the undersquare direction, and performance engines can be a bit oversquare (since it enables higher peak RPM), but on the whole, most engines are fairly square(ish).

 

[Sai Bhagavan]

Well I don't have the head design yet but I have the valve opening and closing timigs for both intake and outlet.Cam profiles are yet to be designed. But my professor asked a tentative power torque graph so had to do this.

 

[cjl]

You can make a decent guess. You've got some weird combinations going on there though - 7.8L flat 6 with a 7500RPM redline and a 117.89mm stroke gives a mean piston speed of around 29.5m/s. This is extremely aggressive even for a racing engine - even Nascar and F1 usually run 2-3m/s slower than this. However, you're only making 450hp, which is a very mild level of output for a 7.8 liter engine making peak power at 6500RPM, and you're only making 360 foot pounds which is also very mild for a 7.8. Granted, you'll make more torque lower down, but still, a 7.8 with that kind of piston speed should be making upwards of 800hp.

 

[Sai Bhagavan]

You can make a decent guess. You've got some weird combinations going on there though - 7.8L flat 6 with a 7500RPM redline and a 117.89mm stroke gives a mean piston speed of around 29.5m/s. This is extremely aggressive even for a racing engine - even Nascar and F1 usually run 2-3m/s slower than this. However, you're only making 450hp, which is a very mild level of output for a 7.8 liter engine making peak power at 6500RPM, and you're only making 360 foot pounds which is also very mild for a 7.8. Granted, you'll make more torque lower down, but still, a 7.8 with that kind of piston speed should be making upwards of 800hp.

I wanted the engine to be naturally aspirated. I was free to choose the parameters and I solely designed the whole engine around the power it is intended to make and the configuration. Yeah it seems a bit bizzare.

 

[cjl]

Yeah, but even naturally aspirated, a 7.8l spinning 7500rpm should be making 800+ horsepower. Why such a large engine? For a 6 cylinder revving that high, you realistically wouldn't want any more than 4.5l or maybe 5l, and you could even keep the horsepower about the same at that engine size.

 

[Joe591]

You should put the engine on a test brake/dynomometer. It will give you a very accurate Torque/rpm graph and I imagine most modern ones will give a power curve along with it.

 

[Sai Bhagavan]

You should put the engine on a test brake/dynomometer. It will give you a very accurate Torque/rpm graph and I imagine most modern ones will give a power curve along with it.

Sir the problem here is am trying to design the engine from ground up. So engine is till in design phase.

 

[Sai Bhagavan]

Yeah, but even naturally aspirated, a 7.8l spinning 7500rpm should be making 800+ horsepower. Why such a large engine? For a 6 cylinder revving that high, you realistically wouldn't want any more than 4.5l or maybe 5l, and you could even keep the horsepower about the same at that engine size.

The compression ratio I used is a more traditional 9.5 so that the engine will run on normal road going fuel and not on higher octane fuels. My calculations landed me at 7.8l capacity.

 

[cjl]

What calculations did you do?