Technical

In-cycle speed variation

On a single cylinder two stroke internal combustion engine, the fuel/air mixture (combustion gases) is ignited every time the piston goes through top dead centre.

This means that the piston and crankshaft rotation are accelerated during the downwards stroke.

During the upwards stroke, the piston has to compress the combustion gases, slowing the piston and crankshaft rotation a little bit.

This variation in crankshaft rotation speed is known as the "in-cycle speed variation".

The crankshaft is rotating fastest near bottom dead centre (BDC) and slowest near top dead centre (TDC).

This is a minor effect and can be mitigated by increasing the weight of the crankshaft/flywheel and reciprocating masses, or by adding more cylinders with symmetrically opposing firing sequences.

However, when you get down to it, the incycle speed variation means that the port time areas, of the transfer ports in particular, will be slightly less than if calculated without taking into account the effects of incycle speed variation.

In the case of the NSR250R, both big end pins are in line with each other on the crankshaft, yet the barrels are at 90º to one another.

As with most motorcycle engines, the crankshaft spins in the same direction as the back wheel (when the bike is moving forwards)

This means that the top left cylinder fires first and the the front right cylinder fires 90º later, with 270º of rotation left until the top left cylinder fires again.

So in this situation, the maximum crank rotation speed probably occurs when the front right piston is near BDC; 270º though a rotation.

The minimum crank rotation speed probably occurs when the top left piston is near TDC; 0/360º through a rotation.

So the port time area calculations for each cylinder will differ slightly due to the incyle speed variation of the crankshaft rotation.

Again, this is a minor effect and typically would be completely ignored.

Petrol

The info below is taken from Wikipedia.

In most countries, including Australia and all of those in Europe, the "headline" octane rating shown on the pump is the RON

Anti-Knock Index (AKI)

In Canada, the United States and some other countries, like Brazil, the headline number is the average of the RON and the MON, called the Anti-Knock Index (AKI, and often written on pumps as (R+M)/2). It may also sometimes be called the Pump Octane Number (PON).

Difference between RON and AKI

Because of the 8 to 10 point difference between RON and AKI (noted below), the octane rating shown in Canada and the United States is 4 to 5 points lower than the rating shown elsewhere in the world for the same fuel.

Octane rating or octane number is a standard measure of the performance of a motor or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating. In broad terms, fuels with a higher octane rating are used in high-compression engines that generally have higher performance. In contrast, fuels with low octane numbers (but high cetane [Hexadecane C16H34] numbers) are ideal for diesel engines. Use of gasoline with less octane numbers may lead to the problem of engine knocking.

Research Octane Number (RON)

The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane.

Motor Octane Number (MON)

There is another type of octane rating, called Motor Octane Number (MON), or the aviation lean octane rating, which is a better measure of how the fuel behaves when under load, as it is determined at 900 rpm engine speed, instead of the 600 rpm for RON. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, higher engine speed, and variable ignition timing to further stress the fuel's knock resistance. Depending on the composition of the fuel, the MON of a modern gasoline will be about 8 to 10 points lower than the RON, however there is no direct link between RON and MON. Normally, fuel specifications require both a minimum RON and a minimum MON.

Effects of octane rating

Higher octane ratings correlate to higher activation energies: This being the amount of applied energy required to initiate combustion. Since higher octane fuels have higher activation energy requirements, it is less likely that a given compression will cause uncontrolled ignition, otherwise known as autoignition or detonation.

The compression ratio is directly related to power and to thermodynamic efficiency of an internal combustion engine (see Otto-cycle). Engines with higher compression ratios develop more area under the Otto-Cycle curve, thus they extract more energy from a given quantity of fuel.

During the compression stroke of an internal combustion engine, as the air / fuels mix is compressed its temperature rises (PV=nRT).

A fuel with a higher octane rating is less prone to auto-ignition and can withstand a greater rise in temperature during the compression stroke of an internal combustion engine without auto-igniting, thus allowing more power to be extracted from the Otto-Cycle.

If during the compression stroke the air / fuel mix reaches a temperature greater than the auto-ignition temperature of the fuel, the fuel self or auto-ignites. When auto-ignition occurs (before the piston reaches the top of its travel) the up-rising piston is then attempting to squeeze the rapidly expanding (exploding) fuel charge. This will usually destroy an engine quickly if allowed to continue.

Octane is a hydrocarbon and an alkane with the chemical formula C₈H₁₈ (n-octane), and the condensed structural formula CH₃(CH₂)₆CH₃ . Octane has many structural isomers that differ by the amount and location of branching in the carbon chain. One of these isomers, 2,2,4-trimethylpentane [iso-octane, (CH₃)₃CCH₂CH(CH₃)_2] is used as one of the standard values in the octane rating scale.

iso-octane (a branched isomer of octane) by definition has a RON and MON of exactly 100.

heptane (also called n-heptane) by definition has a RON and MON of exactly 0.

octane (also called n-octane) has RON of -10. (negative ten)

Squish and Combustion Volume

The squish and the combustion volume are two very important two stroke tuning parameters.

The 125cc two stroke cylinder has had much development work over the years due it being the size of cylinder commonly used in 125cc, 250cc and 500cc two stroke motorcycle GP engines, with a few exceptions.

Likewise, the NSR250R has two 125cc cylinders.

Information on the squish and combustion volume of the NSR250R comes from individuals taking measurements in their home garages and workshops and will probably be subject to slight variation as a result.

By placing a length of solder wire down the spark plug hole and squishing it with the piston, by rotating the flywheel by hand, it is possible to get a good idea of the squish by measuring the width of the squished solder wire with a set of vernier callipers.

Squish is typically 1.1 to 1.2mm for a standard NSR250R

It can vary depending on the thickness of the base gasket and head gasket. Also, it is not known if the dome angle and shape of the piston perfectly matches the geometry of the head so the squish may vary in depth radially.

Measurement of the combustion volume is usually done using a burette/pipette to fill the cylinder with low viscosity oil, up to the top of the spark plug hole, when the piston is at TDC.

The combustion volume of a standard NSR250R is 13.3cc

HRC F3-TT specs reduce the squish to as low as 0.8mm. Note that below 0.65mm there is a risk of the piston hitting the head due to conrod/crankshaft deformation under stress.

HRC F3-TT specs reduce the combustion volume down as low as 11.5-11.7cc.

These specs are probably very close to the Honda RS250 race bike specs.

Increasing the compression typically reduces the rpm at which peak power is produced.

This rpm level can possibly be increased again by altering the ignition timing, but an excellent understanding of two stroke tuning is required to get the balance right in terms of performance and reliability.

NOTE: the grade of fuel used becomes critical, when squish and combustion volume are reduced. Getting it will wrong will result in detonation.

Other variables also come into effect, especially carburetor jetting and ignition timing.

Using 98 RON (pump gas) or 100 MON (Leaded Avgas) becomes compulsory in order to prevent detonation when increases in the compression and ignition timing become too extreme.

The Expansion Chamber

The expansion chamber is typically the exhaust pipe. It runs from the exhaust header on the barrel to the muffler.

The purpose of the expansion chamber is to use sonic harmonic resonance to:

• • Help extract the burnt exhaust gases from the cylinder

  • Help stop fresh unburnt fuel/air from exiting the exhaust port

A correctly shaped expansion chamber can add approximately 50% more power to a two stroke engine, within a specific rev range, compared to a straight pipe.