Sunday, July 5, 2015


The relationship between torque and power is poorly understood by the majority of people.
I gather this as people usually say "I've got the 130bhp TDI" ... or the "I've got a 700nm turbo gasoline engine".

So lets start with the Fuel as this actually separates everything- Gas vs Diesel :
So; the way fuel burns is very important for you to understand the engine you have. 
Gas engines input air and gas (the so called mixture) into the cylinder, and then compress it all (normally at a 11 to 1 ratio) to a point when the mixture is ready to blow... a small spark triggers this.
The point here is that the mix is ready to blow while it is getting compression and this adds to the "explosive effect". The more RON (or octanes) the fuel has, the better it will resist compression and not detonate. When the fuel detonates prematurely you get the auto-detonation effect that can hurt the engine... it also produces a high pitch PING on the engine as ALL the mechanical parts involved into the compression are pressed against each other and actually squirt the oil between them dry.
So this mix is being compressed and ready to explode... and the higher it resists the more voilent the explosion when it happens... and that is why high compression and high octane fuels will produce more power, quicker and with much more heat.

HOWEVER... there is a limited amount of time an explosion can happen, this will vary with the violence but in the end, as the engine accelerates, the explosion will have less and less time to occur. 
So a Gas engine will have a variable "timing" advance on the spark... this basically means that, the faster the engine needs to spin, the faster the timing advance will be. 
The other part of this equation is the AFR... the Air-Fuel-Ratio will regulate the amount of air to the amount of Fuel in the mix.
So Mix 101 - a Perfect mix is called stoichiometric mix.  It is composed of 15 grams of air to 1 gram of fuel. This is called the perfect mix as it will take 4 to 5 milliseconds to burn. So a normal engine running at 6000 rpm, would have a full rotation in 10 milliseconds, meaning a full piston stroke at 5 milliseconds, in turn, meaning that the mix would explode fast enough. However due to the extreme heat generated by this type of burn and the simple fact that not every one runs pure race fuel, it is rarely used. 
Normal fuels produce a stoichiometric mix at 14.1:1 because of all the crap... sorry... "additives" in it... a pure octane fuel would run at 14.7:1 pushing it to the near ideal formula.
The mix in a normal gas engine ranges from 12-13.5:1 depending on the fuel and the ability of the ECU to decide. This is called a rich mixture and will generate MORE power... it will however take a bit more time to burn and do timing advance is needed.
The ECU will vary this, obviously, and the consequence is as much as 35degrees advance BTDC (before top dead center...or before the piston reaches the top of it's movement and stops before turning back).
There is also something in the timing equation called inertia... sometimes the engine doesn't advance as much as it would require because the excessive pressure while it compresses the exploding gas, could turn the engine to a backspin... The rotation of all the engine parts produce a positive spin inertia that helps countering this effect, but the less speed it has, the less inertia it has. That is why it is so dangerous to advance the ignition at low revs, especially and a high efficiency engine (or a cross plane crank one).

So... it should be clear by now that the explosion of the gas-air mix is violent and quick... but it happens once. That is one of the reasons for the gas engine to have low torque and high rpms.

Now the diesel engine ... or so called " the fuel of the devil" to a true motorhead:
The diesel engine works a lot like the gas engine... pistons, valves, motion... and that's about it! 
The Deisel engine does not input a MIX of air + diesel! It intakes JUST the fuel. 
So as a consequence, the diesel engine can compress way beyond 11:1... it typically compresses around 18 to 20:1.... it's just air so it will not explode on it's own. 
The explosion comes from the injection of fuel direct into the piston head... creating a controlled, phased detonation that is kept while the fuel is being injected into the piston. 
That is why the diesel pistons have that half-donut like groove, with the cone like "spreader"... so that the injected fuel explosion is redirected and spread across the entire chamber. 

This means that while on a Gas engine the explosion is one, once and violent... on a diesel, the explosion keeps happening ALL THE WAY down the piston course, and since the diesel is basically just oil, the explosion is much less violent...and in consequence slow as hell. 

Facts time about the fuel type on your engine:
The Gas engine has a single, violent and fast explosion... making the engine generate less torque, but also making the engine able to generate a lot more RPM.
The Diesel engine has several slow, lower energy explosions all the way down the piston course...generating loads of torque but not being able to do that quickly enough to generate high RPMs.

The first conclusions?
If you are buying a diesel car, that you really do not care about the bhp, but rather the torque curve on your engine.

If you are buying a gas engine car, that the torque you want is the torque to handle the loads you are going to press the car against... it's weight and losses due to transmission. Because what you really want it that torque to be able to be translated into BHP by means of rpms... so yeah you want some torque... but not too much, as that will generate wheel spin...but you really want high rpm with high bhp at high rpm. 

If you are pulling a caravan, do not use the gas just doesn't have the torque without having to become MASSIVE in size.
If you are racing a car, do not use the diesel was not built for generating power on high rpms... and you don't race at 2000rpms!

Now for the second part... engine design, torque and the so very important LPS... and a touch of valve fluctuation.
Lots of people do not understand the redline on their engine, why it exists and why does the 2.0 version of the car rev 1000rpm more than the 2.2 version of the same engine on the same car.

Engines basically turn linear motion into rotative motion. They do so by connecting a linear working part (the piston) to rotating part (the crank shaft), with a rod. 
Having this clear let's go back to the physics class most weren't paying attention to: moment=f x rod
IF you try to open a door, grabbing the door 2cm from it's pivot point, and then do the exact same thing but using the handle (a good 80cm from the pivot point), you will notice that you have to effort a LOT less by moving the force away from the pivot point. 

Now apply this principle to the rotation of the engine assembly you see above: IF the point of the connecting rod on the crankshaft, moves away from the pivot point (the center of the crankshaft and flywheel), then the effort needed to turn it is far less. 
Since the rod pulls and pushes the piston, the bigger the distance, the bigger the rod and as a consequence the longer the travel the piston will turn.

So by now it should be clear that a piston that travels a longer run, will be able to produce a better moment, or a better rotation force... and that is torque? yup... it's the rotation force.

Now enters another part of the physics on an engine and that is the effects of acceleration and deceleration on materials... the so very important INERTIA. 
Inertia is a lady you all should learn to respect as it ultimately can kill you in many more ways that it will give you pleasure.
Now inertia says that, putting something in motion requires more energy than just maintaining that motion.... just the same as countering that motion requires a lot more energy than maintaining that motion. And the heavier the thing is... the worse. 
So picture you have a bank safe on a skateboard. Making the safe move will consume a lot of energy, but once it is moving it is easier to pull... then something gets in your way and you need to stop the safe from moving... now that is something to experience as you will have to press really hard agains the movement of the safe to stop it. 
You see, a piston, ultimately is metal, that ultimately is a bunch of molecules of several metals and carbon mixed together. Making metal move, will generate stress on the molecules as they are "glued" together.... so if the piston has an anchorage point (where the connecting rod links to it), the remaining parts of it will only be linked to that point by the molecular bond on the metal. 
If the piston moves up and down very fast, every time it stops and turns back, the majority of it's metal will try to continue the movement it already has and this generates stress. 
Generate too much stress and you will crack it... continue and you will disintegrate the piston:
So there is only so much a piston can handle in terms of pressure.

You also need to understand that the pressure of the piston against the cylinder walls, generate drag that will then generate heat! The faster the travel the more heat is generated... too much heat and:
... it melts!

So a pre-summary:
The more a piston travels, the more toque it generates.
The faster a piston travels, the more chance it has of melting.
The more it changes direction at speed, the more change it can crack.

The engine design part:
There are 3 types of design on the engine... 
The SQUARE engine -  when the diameter of the cylinder is equal to the length the piston runs inside the cylinder.
The OVER-SQUARE engine - when the diameter of the cylinder is bigger than the length the piston runs inside the cylinder.
The UNDER-SQUARE engine - when the diameter of the cylinder is smaller than the length the piston runs inside the cylinder.

Now comes in the LPS the Linear Piston Speed -> this is a means to measure the speed that the piston travels distance inside the cylinder. This keeps changing from 0 to well over 30meters/second back to 0 ans the piston accelerates and decelerates back. 
Since this varies too much, a MPS equation is set... MPS is the Mean Piston Speed. it is calculated as, MPS = 2 x stroke length(in meters) x rps(rotations per second.... or RPM/60).
An normal engine will run a maxmimum 22m/s MPS.
An high spec street engine will run 25m/s and be able to withstand 28m/s for seconds before starting to have structural damage at metal grain level.... A.K.A starting to micro-fissure.
An racing engine will run 28m/s to 29m/s and peak at over 32m/s (but will show fissures at the end of the race)
An very light racing engine will run 35m/s...but will have catastrophic failure within hours (if not minutes) 

Since an OverSquare engine makes it's volumetric capacity by having large bores on the cylinders, but very little distance to stroke, it's pistons run less distance per rpm... allowing you to pull the RPM limit further (assuming the fuel allows you to). 
On the other hand, an UnderSquare engine makes it's volumetric capacity by having a long stroke. This forces the piston to run longer distances in shorter time per rpm... so if you push-it... it will crack and/or melt the skirts.

So having ALL this into account, you have to choose one out of 2 things:
either you have a OverSquare engine that produces lots of RPMs while still within the safe 25m/s range...ooooor you have a UnderSquare engine that will produce better torque but will reach the 25m/s MPS limits at a lot lower rpms.

This is why the Honda F20C engine redlines at 9000rpm, and the Honda F22C engine (equal in everything except a longer stroke that gives him an extra 200 cc) will redline at 8000rpm.

Now is time for another pre-summary:
IF your engine redlines as 5000rpm, do not pull it beyond that mark for over 2 or 3 seconds... you will be generating micro-fissures in the pistons and over wear the pistons skirts... in time, those will become full blown cracks and will lead to catastrophic failure.

If you engine produces less torque and you want to increase it, the healthier way is by stroking the engine, BUT if you do so, then LOWER the RPM limit.

If you are boring the engine to have more displacement, then DO NOT increase the RPM limit... you can only do that if you reduce the stroke!

And the very important: if you lower the engine stroke to be able to have more RPM's, then you also need to reinforce the valve springs and eventually get lighter valves. The Valve fluctuation phenomenon will happen if they are forced to work at higher RPM's that they where engineered for... with will eventually lead to a piston touching the valve and kaboom... it can bend the valve or/and crask the piston. 

A lot of people say that HP doesn't exist... but they are wrong. It does... what it is, is a byproduct of torque at engine speed. 

The formula is : HorsePower = (rpm x torque)/5252

So the HorsePower, being that the 5252 is constant can be said to be the torque at speed of the engine.
And this is where the idiots that like to say that "a torque wrench has more torque than a honda engine" will have to eat their lack of knowledge for lunch.
The way an engine produces torque is limited in the way it is designed and the fuel being used. 

A long stroke 1.9 diesel engine will produce torque as low as 1000rpm, peak at 1800rpm with 310nm not sustain it and slowly decrease and then die at 2500rpm..allowing to push to 3000rpm with some HP.  This means 2000 usable RPM. 
A long stroke 2.0 turbo gasoline engine may generate a decent toque figure at 2500rpm, peak at 3500 rpm sustain for 500rpm with 360nm, and decrease with decent figures till 5500rpm and then redline at 6000rpm. This means 3500 usable RPM
A shortstroke 2.0 non turbo engine will generate lower torque levels but a decent level at 3000rpm, then peak at 6000rpm with 202nm, sustain the torque till 8000rpm and decline to the 9000rpm redline. This means 6000RPM of usable engine.

There is also a catch here! The diesel engine produces 130 HP, the 2.0 turbo gas engine 265 hp and the 2.0 N.A. engine 240hp. 
However, the longer stroke on the turbo, gas engine, means that the MIX explosion time will not happen in a perfect manner... a lot of the explosion force gets pushed into the cylinder walls instead of directly into the piston, at the most "vulnerable" time in the gas expansion... and this gets worse as the piston goes down and the explosion is already past the peak. 
That long stroke does have a better effect on the sustained diesel engine explosion than it has on an one time explosion of the gas engine... and that is why the torque figures drop faster in a longer stroke engine than a short stroke one.

Is there a lesson to learn from? yup! torque should be matched to the weight the car needs to pull and the mechanical loss involved in pulling it (gearbox and traction system)... then the torque curve needs to be set in order to have the maximum possible engine for the longest possible time.. and since while racing, you lose time every time you shift, there is a considerable advantage to have more rpms...

And this is why the following video shows 2 cars of equal engine HP but different weight and torque figures... they both have 250BHP, the TURBO car has 9% plus weight to a 45% plus torque figure... but a very crucial 3000rpm LESS than the N.A.

So is the Honda better? Engine wise... yup!
chassis and usability is another thing... that brings even more variables into the mix. They are both front wheel drive cars, DURING a corner, the N.A. engine will generate less torque and this means that it will be able to step on the gas heavier and earlier without upsetting the traction as much as the excessively high torque figures the megane has would allow... this of course is without taking into consideration the torque biasing diff on the megane that would even this by cutting the engine power and actually sending more torque to the wheel with the better grip.
So purelly mechanically speaking the excessive torque would be a problem in the megane... out of the corner, the megane would pull with better acceleration on the same gear while the honda driver would have to be either pulling the same gear and using the 3000rpm on VTEC, or shifting down to keep it within VTEC range... so this would depend on the track... if the track would have climbs out of corners, thought, the megane would clearly be in it's kingdom (and this partially explains the ring time it has for marketing).

The Summary:
Torque is a way or expressing the engine capacity to pull weight. It can be generated in the form of sheer weight (like a truck that needs to have a diesel engine) or multiplication through gears, as the standard everyday diesel car has.
Given the right conditions, you want your engine to have more RPM and high power at high RPM than a high figure torque at low RPM... This becomes more evident at speed in a high gear, as an engine built for low end torque is out of it's element as the air being pushed in front of the car gets more difficult to cut through. That is when you need the torque that was really big in the beginning but is now long gone...and then you need to shift, because you are out of engine!

This is why the best tarmac motorsports cars use very high revving engines like F1 18000 rpm (some used to have 20000rpm, DTM 9000rpm (some with 12000rpm), BTCC 8500rpm and so on. the usability on those engines is much better that with more torque and less rpm.

Rally, with their heavy 4wd systems and very difficult terrains, on the other hand, are all limited to 6000rpm and a peak of 300bhp to 340bhp at 5000rpm. This is where you really need toque as the 4wd car passing a muddy up-hill will height twice as much... at least.

No comments:

Post a Comment