An Engine Question

Acolyte

Acolyte

Acolyte

Soldato
Joined
18 Oct 2002
Posts
3,803
Location
Bristol
Hi,

This question has bugged me for a while but im sure you guys will be able to tell me the answer.

Take for example the engine in my car, it produces peak power of 137hp @ 6500RPM, it redlines at 7100RPM.

WHY does the car not produce its maximum power at the 7100RPM?

Why does the car produce less power after 6500RPM? Take for example the speed 6800RPM, it is spinning faster than 6500RPM, it is burning more fuel, therefore should it not be producing more power than it is at 6500RPM?

I dont get it.

Cheers.
 
It all depends on the timings that your cams give your engine. For example at 6500rpm the timings are just right for the rpm so it is the most efficient at that rpm.
 
Horspower is a function of torque and the engine speed. As the torque curve starts to drop off the horsepower continues to rise until the torque figure is low enough that it starts reducing the horsepower figure relative to revs. That's the tech answer.
 
As has been said, it's due to the max torque not being at the max rpm. A handy feature of this is that it allows you to get up hills...
 
I asked my friend - And the answer he got from another car forum was:

Because the Volumetric efficiency of the engine is droping off faster than the Revs rise. As rpm rises, u have less time to fill the cyinders properly.
 
Virdi said:
I asked my friend - And the answer he got from another car forum was:

Because the Volumetric efficiency of the engine is droping off faster than the Revs rise. As rpm rises, u have less time to fill the cyinders properly.
Click to expand...

So if this theory is correct then would bigger injectors give you the ability to get a BHP at even higher revs with out it dropping off?
 
In simple terms:

Remembering that power is work done per unit of time. As engine speed increases then the mechanical bits are moving further per unit of time. If everything is moving further then more energy is lost to friction etc.

Eventually the increase in losses for a given engine speed increase becomes greater than the amount of energy release associated with that speed increase so overall power output actually falls.
 
Virdi said:
I asked my friend - And the answer he got from another car forum was:

Because the Volumetric efficiency of the engine is droping off faster than the Revs rise. As rpm rises, u have less time to fill the cyinders properly.
Click to expand...

One way of getting over this is overlapping the valve openings so the intake valves open before the exhaust ones close so it draws the mixture in so to speak. This is the principle behind vtec, this is why the max power is so high up the rev band as the secondary cams are designed to operate at high revs but won't at low ones.
 
Its all to do with the timings within the engine.

Changin the cams can increase horsepower by adjusting the timings, and allowing it run to a higher rpm. However this will be at the cost of low down power.

Its whats great about a Vtec system and why it can rev so high. You have a cam profile for low down power, then another cam for high end power.
 
Bug One said:
Its all to do with the timings within the engine.

Changin the cams can increase horsepower by adjusting the timings, and allowing it run to a higher rpm. However this will be at the cost of low down power.

Its whats great about a Vtec system. You have a cam profile for low down power, then another cam for high end power.
Click to expand...

I read somewhere that Volvo invented something like Vtec before honda did :confused:

It had a lot of power down low and had a lot of power up high
 
Zip said:
I read somewhere that Volvo invented something like Vtec before honda did :confused:

It had a lot of power down low and had a lot of power up high
Click to expand...
I have variable valce timing on my Fiat. Lots of manufacturers use it now.

However I think most manufacturers do it to create more low down grunt, where Honda have designed and marketed it well to create high end power.
 
Bug One said:
I have variable valce timing on my Fiat. Lots of manufacturers use it now.

However I think most manufacturers do it to create more low down grunt, where Honda have designed and marketed it well to create high end power.
Click to expand...

It was called the H engine.
Or something like that.
The only problem is something made it a bit heavy but i cant remember what.
Its on wiki if you want to look it up:)
 
Zip said:
So if this theory is correct then would bigger injectors give you the ability to get a BHP at even higher revs with out it dropping off?
Click to expand...

A lot of it is to do with the 'choking' of the flow into the engine through the intake valves. With any valve opening there is a maximum amount of flow that can happen before this occurs.

So they may help (I assume you mean injectors which can inject more fuel at a time) but you would need to take into account more than just that to work it out.
 
Bigger injectors won't help at all, unless they are actually running out of injector duration (won't be happening on a standard engine). Air flow is the primary reason for the drop in torque at high RPM, you have to get a chunk of air through a relatively small hole very, very quickly. At 7000RPM you only have around 4 thousands of a second for the air to get into the cylinder. This is why the design of the cylinder head is critical to engine performance.
 
Dogbreath said:
Bigger injectors won't help at all, unless they are actually running out of injector duration (won't be happening on a standard engine). Air flow is the primary reason for the drop in torque at high RPM, you have to get a chunk of air through a relatively small hole very, very quickly. At 7000RPM you only have around 4 thousands of a second for the air to get into the cylinder. This is why the design of the cylinder head is critical to engine performance.
Click to expand...

All true.

Remember, kids - air is a fluid ;)

*n
 
Dogbreath said:
Bigger injectors won't help at all, unless they are actually running out of injector duration (won't be happening on a standard engine). Air flow is the primary reason for the drop in torque at high RPM, you have to get a chunk of air through a relatively small hole very, very quickly. At 7000RPM you only have around 4 thousands of a second for the air to get into the cylinder. This is why the design of the cylinder head is critical to engine performance.
Click to expand...



Yes, but F1 engines manage it at 10k rpm or more. I still say that this is deliberate, not a side effect of the engineering.


M
 
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