By Stefan Boshoff, Country TRAX Instructor – Free State

Power and Torque – Part 1

Winter is upon us. Here in the Free State we are first to realize the fact, so if it has not arrived in your village we can assure you that it’s on the way. Winter is a time when biking activity tends to slow down a little, except in Kwa Zulu Natal where there is one day of winter a year…. Doing some training in winter is a great idea of keeping active on your bike. Days are not as hot as in summer which makes for a more comfortable time on the bike and our venues all have fireplaces to keep you warm in the evening whist you exchange wisdom gained from the day’s training. Have a look at www.countrytrax.co.za.

This month we tackle a topic which is widely debated around braai fires and internet forums, and we realized from listening and reading that a significant number of riders debate the issue at “high” levels without understanding the fundamentals. Hence this attempt to explain Torque and Power and their relationship very briefly.

The basic concept of any 4 stroke engine is shown in the picture. The piston moves down the barrel, sucking a mixture of air and fuel through the inlet valves. The mixture is compressed as the piston moves back up with the valves closed. The spark plug now ignites the mixture and the “explosion” results in expanding gas forcing the piston down again. Upon return, the gas leaves the barrel through the exhaust valve. The problem that engine designers are faced with, is that mathematics have not changed since the days of Einstein, and therefore the characteristics of an engine today are exactly the same as they were since inception of the idea. The force that the expanding gas transfers to the piston goes through the connecting rod and creates a torque (rotational force) in the crank shaft. Once torque starts moving (spinning), it becomes power, and the only way you can make more power is by taking the torque you have and spinning it faster.

Just to make sure we understand the maths…. Force (N) = Mass (kg) x acceleration (m/s²). The mass of the piston is accelerated by the expanding gas, causing a force. Torque (Nm) = Force (N) x distance (m). The distance we refer to here is the measurement from the centre of the crankshaft to the centre of the connecting rod, or half of the stroke length. Power (kW) = Torque (Nm) x Rotation rate (rpm) / 9550. Everyone trying to improve the modern day engine is stuck with these mathematical truths, and there is nothing we can do about it. We can vary the bore and stroke for a given cubic capacity. A longer stroke is a good idea for increased torque, but that brings about two other challenges. For the same cubic capacity, you now have a smaller bore, which gives you less force. And the piston velocity increases because it now has to travel through a longer distance per one revolution. There are limits to piston velocity which are dictated by the materials they are made of.

An important variable that assists in generating force on the piston is known as “compression ratio”. This is the ratio of the volume of gas in the barrel with the piston right at the bottom and the volume when the piston is at top dead centre (or right at the top of the stroke). In other words, it determines the amount of pressure generated during the compression stroke whilst the gas mixture is compressed just prior to ignition. The higher the pressure, the greater the force generated. Over time, as materials developed we see a trend of increased compression ratios in engines, which is the biggest single factor resulting in increased torque specifications for a given cubic capacity and optimal fuel / air mixtures. The higher pressures unfortunately cause increased stress on engine components, and we have to live with the decline in reliability – unless better materials of construction are utilized.

So let’s get practical. Below is a table with some of the modern adventure bikes’ specifications. Just for good measure, we also added an old BMW R80GS and two mighty superbikes namely the Yamaha R1 and the new BMW S1000RR, just to prove the maths.

What we have calculated, is a value we call “specific torque”. This value is the torque generated per 1000 cc of capacity, divided by the compression ratio. It proves that all the engines listed generate the same specific torque within 5% of the average value.

We have seen that for a specific engine configuration, we can only generate power (kW) by spinning the torque faster. Just look at the rpm values for the huge kW’s that the superbikes generate. Also notice the increased compression ratios for the engines with higher torque. A good example is the development of the GS over the years. The old R80GS had a compression ratio of 8.3, compared to the 1200 GS with a CR of 11 and the new 2010 model up to 12. Now you know where the increased torque comes from. From all of this, you can make a very good guess of the maximum torque value for any engine if you know the cubic capacity and compression ratio. In part 2 we take a deeper look into power and torque curves and what we can learn from reading them.