Re: Horsepower / Torque [ On track difference ]


From what I've learnt, there is link between rpm and fuel burning time. As rpm is increased, fuel ignition time is advanced to allow enough time for fuel burn. Though it's fixed for carb engines, modern ecu equipped engines alter spark timing along with valve timing for different loads and speeds. The terms rpm and piston speed are different and independent. There is a classification based on piston speed and for street bike, it's around 22m/s I think and this varies for each category. The thing is, long strokes have higher piston speeds compared to short strokes for a particular rpm. This prevents long stroke cruisers to stick to low rpms. This low rpm is compensated by higher torque output..
Though this is completely based on my understanding, corrections are welcome
Sent from my ST18i

Re: Horsepower / Torque [ On track difference ]

Brother I appreciate your quote. But 25 m/s is the average mean speed of a piston at say 3.5 to 4k rpm. Also ecu does not control variable valve timings. If you short it, it will be VVT a sticker that you can see on a Hyundai cars. Also a/f mixture is controlled by ecu but with the help of various sensors. Majorly a O2 sensor. Which is already not present on my tbts500. Which means ecu is doing it basis a pure feeded map.
Basis this I can understand a rich a/f mixture in starting rpm results from in low power, but I have still not understood as to why torque peaks before power, whereas both are correlated to each other.

Thanks in advance.

Re: Horsepower / Torque [ On track difference ]

Modern ecu's control more number of parameters and it does include vvt AFAIK. O2 sensor is used in closed loop fuel injectors. If it's not present, it's an open loop injection system. And yes, open loop system depends only on intake side parameters and operate on premapped basis. For torque, it's the point where fuel is most efficiently burnt(fuel is nearly completely burnt off). Beyond that, as shown in formula, torque reduces slowly but the rpm increases greatly leading to more power. This torque decrease and rpm increase continues till maximum power is achieved. Beyond this point, the torque produced begins to lower in a faster rate compared to increase in rpm. This causes the reduction in both torque and power.
The torque reduces due to the fact that fuel is burnt inefficiently. This is due to factors such as burn time, valve opening limits, air intake drag, back pressure, etc. This limits the maximum rev of the engine as it becomes too inefficient to operate beyond that rpm(along with harmonic vibrations). Inorder to minimise these limiting parameters, 4 valves(more intake, exhaust area), vvt, turbos, etc are used..

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Re: Horsepower / Torque [ On track difference ]

Seriously am liking it.
Agreed closed loop systems use many sensor, but o2 is the sensor which is main in deciding what a/f mixture into engine. Also for open systems like one in my bike they use more of a generalized map. Only reason why you will find always find blackened spark plug. Basis it send rich mixture during start which is roughly initial few km. I buy your point that due to non proper burning fuel(still not 100%) torque goes down as rpm goes after a point. If it is so than why is power always peak after torque does. I.e when power is peaking after torque is decreasing according to formula power=torque x rpm power should also go down. And also difference of rpm between two is higher in short stroke engines.

Ant explanations are welcome. Thanks in advance.

Re: Horsepower / Torque [ On track difference ]

Nope...O2 sensor alone is not important. A MAF sensor is used to detect the air flow into the engine. This allows the ecu to adjust fuel accordingly though it's not completly accurate. See, power is the product of torque and rpm. So if power should go down, torque and rpm should go down. The reducing torque in small quantity is being replaced by larger quantity of rpm. This prevents the reduction in power and helps increase it due to enormous rpm. It is only when the rise in rpm is less in quantity compared to rate of decrease in torque that power begins to fade. Beyond max torque point, torque begins to fade. But this rate of fade increases with increase in rpm and beyond certain point, begins to fade at a rapid pace, causing loss in power.
A short stroke engine produces less torque but can rev more. As in general note, power is higher at higher rpm,so these engines produce high power but low torque(short stroke). Tuning these engines help them reduce the torque decrease rate and achieve high rpm and thus high power but generally they have a narrow powerband.

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Re: Horsepower / Torque [ On track difference ]

That explains a lot.
Thanks bro

Re: Horsepower / Torque [ On track difference ]

I've an OT question in the line of sensors.
I stumbled on to this thread Why remove the O2 Sensors? | Victory Motorcycles: Motorcycle Forums Victory cruisers' forum.

To summarize: O₂ sensors on exhaust help make AFR leaner to meet new emission norms.

This may not be true with motorcycles available in Indian market. However, it still raises a question: Are closed-loop FI in below 300cc bikes (in India) give real-time feedback for AFR change or to a pre-mapped AFR?

Re: Horsepower / Torque [ On track difference ]

O2 sensors help to ensure maximum possible combustion by sensing the O2 level in the exhaust.
More oxygen= low fuel content and so ecu increases fuel.
No oxygen= excess fuel and so ecu reduces fuel.
In this way, the leanest mixture possible without knocking is achieved. Generally, closed loop systems are real time based systems with predefined algorithms thats handled by the oxygen sensor. I don't know about these programming styles but if present, mapping or algorithm based depends only on cost of fi systems.

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Re: Horsepower / Torque [ On track difference ]

Normally most of closed loop systems make there Maps real time. Based on the Sensor feed backs.

RPM and Torque!

Hey guys, I know the basics of Toque and Power and how they are related to the engine RPM. But even after a lot of googling I am unable to find an answer to this question. What is the phenomenon by which in an internal combustion engine when the torque is not enough to handle the load and no matter how much you open the throttle, the engine RPM will not go up?

I will explain my question a little bit - Basically when you twist the throttle/accelerator the carburettor MUST supply a rich fuel air mixture to the engine causing the RPM to go up. So when you are going uphill in a higher gear and the engine is unable to provide the required torque, after certain point no matter how much you open the throttle the RPM does not go up and the vehicle cannot accelerate, and at this point you decide to downshift to get more torque. So in this scenario how is it possible that opening the throttle is not causing the RPM to increase? Is it a natural phenomenon or there is some arrangement in all vehicles to cut the throttle when there is not enough torque to handle the load?

Re: RPM and Torque!

Approved and merged!

I'll let the experts answer that in detail.
My understanding is that gearing matters.
HP = Torque * RPM / 5252

Now, HP is usually made at higher rpms than torque.
So, if you're already at a limit where there's no more torque or rpms don't climb, HP will be made less.
Add to this the factor of downward force via gravity. You need enough power and torque to negate gravitational effect.
Also, the amount of energy used in lower gears is less.
Lower gears see more revolutions, hence energy per revolution or power required is less. This makes the climb easier.

Read this - Power and Torque: Understanding the Relationship Between the Two, by EPI Inc.

Re: RPM and Torque!


Let me try explaining this in the simplest way. The word you are looking for is gearing.

So the engine has a peak torque capacity. If the load increases, basically what is happening is amount of effort required to accelerate is higher than the torque the car/bike makes. Downshifting allows for gearing change which basically helps engine cope up with the load.

Have you ever ridden a Geared cycle? If you have, you will realize in lower gears, you have to pedal more to get the same distance but the force on the pedals is less.
In higher gears, you require less pedals for the same distance but the effort required on pedals is more.

Let me put it this way, you covering certain distance is a target work. You can do it in 2 ways: Pedal more with less effort. Pedal less with higher effort. In motorcycles it is the same. The only difference is that sometimes for some work, you just wouldn't have the required effort. So, you downshift to reduce effort but increase pedals. In bike's case pedals is engine RPM.

This is the same with bikes. When you are going uphill in 5th gear and you will see the bike is not climbing, you will see its RPM will drop. Which means it coming below the RPM where it makes peak torque or power and hence is making less torque. When you downshift, the RPM will rise because of lower gearing similarly as in cycle. This results in bike coming back to the peak or higher torque zones allowing you to climb.

I hope it helps you.

Rachit

Re: RPM and Torque!

Its the same phenomenon that doesn't allow a vehicle to reach speeds beyond its max. The total output at wheels must be more than the total opposing force of the vehicle for it to accelerate. At saturation, it maintains it's speed. If opposing forces are more than the total force at wheels for that particular engine rpm, the vehicle can't move forward unless the torque at wheel is increased, ie, gear is lowered. Under this condition, the engine cannot produce more torque than that is available at that rpm. So, throttle has no effect on the engine.

Re: What is "Power", what is "Torque"..??

That pretty much sums it up!

Re: What is "Power", what is "Torque"..??

Thanks Divya, Rachit and John! Your answers definitely gave me a little bit more insight into the relationship between torque, power and RPM. So contrary to what I thought as engine RPM should be always dependent on rider input via throttle, it looks like the torque or power can influence the RPM and override the throttle input. Correct?

Also I guess you can sense that my initial question partly remains unanswered still, which is the mechanical part of it. That is the when the bike is climbing uphill and is losing torque and there by RPM, you are lazy and dont want to downshift yet and give it all to the throttle. Mechanically this should open the butterfly valve and cause more air-fuel mixture to go to engine, right? Or is there any mechanism by which carburettor limits air flow irrespective of the throttle valve position?