Tag Archives: Engines

Track day tribulations

I keep assuring you, my dear readers, that I am not a mechanic. It is about time I write a post that helps illustrate that point.

The last track day I did on the RGV was not the biggest success. The bike was failing to accelerate in the top gears and by the “seat of the pants dyno” was even a little weak in the lower gears. Apart from some cursory inspections and a new set of spark plugs, I hadn’t really done much on the bike since the previous track day, so it was not much of a surprise.  I can’t even say “I should have known better”, because I do know better… As they say: “Proper preparation prevents poor performance”

Anyway, both the bike and I survived the day, so back at home, it was time to work out what went wrong… It didn’t take long – the left carburettor float bowl was leaking fuel. Put simply, the top cylinder was unable to get the fuel it needed when running at full throttle.  The float bowl o-ring had a gap if about 3mm, through which the fuel was able to escape.
Where the carburetors are situated, meant the fuel leak was relatively safe.  The fuel would fall on the crank case, which was hot enough for most of it to evaporate off.

The reason there was a gap in the o-ring was no mystery: I had cut it. Of course, I hadn’t done this without a good reason. At a previous track day I had to take the carburettor apart to clean one of the jets that had become blocked. The o-ring had stretched and no amount of careful prodding, cussing and holding my mouth correctly was going to get the o-ring to fit. One truism of track side maintenance has always been “No matter what spares you have with you, you’ll need something you don’t have.” Faced with the choice of “go home” or “improvise” I chose the latter and cut a small section of the o-ring out.

Why did the o-ring swell? Truthfully, I do not know.  I am guessing it reacted to something, but I could not correlate what the Internet tells me and anything that I remember doing.   I think I may have fitted the o-ring with some rubber grease, but that is designed not to react with it!  Most likely, I got some carby-cleaner on the o-ring and it  reacted to this, although I don’t know for certain.  As I had now discovered, it eventually returned to its normal size, thus leaving me with a gap.

Whilst on the matter of mechanical confessions… That day, when I refitted the carburettor, I broke the thread of the plastic choke nut. These are hollow, allowing the choke cable to pass through them. As a result they are incredibly easy to over-tighten and snap. Without it in place a lot more air would be drawn through the carburettor, causing the engine to run dangerously lean.

Plastic choke nut assembly

There was insufficient thread left on the choke nut to hold against the spring tension, but another RGV owner came up with a clever way of using cable ties to hold the choke nut in place. It was certainly a bodge job, but it easily held up on the day.

So, what did I learn from my mistakes? Lots of things, really!

  1. Do your preparation before the track day, not at it.
  2. If the manual doesn’t suggest using sealant or other consumables, then you probably shouldn’t.
  3. A twenty-four year old bike that you thrash when you ride it needs plenty of TLC/maintenance when you aren’t riding it.
  4. A post track day inspection and service is a good idea.
  5. If you can’t have the right spares with you, at least have plenty of cable-ties!

 

Taking charge of the situation

Recently, my trusty steed (the VFR) has been anything but “trusty”. After a great ride through the Victorian hills it abruptly decided not to start. The all too familiar “chugging starter motor accompanied with the dash going dim followed by the clock resetting to 1:00am” of a flat battery greeted me. Given that the battery would be approaching the five year mark, I thought nothing of it and replaced it.

Five engine restarts with the new battery later and I was left staring in disbelief as the dash again went dim and the clock went back to 1:00am. Sidenote: Why is it that Honda insists on making the clock so impossible to set without uttering profanities? When pressing two buttons at the same time means AT EXACTLY THE SAME TIME!

After coaxing the battery back in to a reasonable state with a charger it was time to whip out the multimeter and perform some testing. The simplest test from the workshop manual consists of running the engine at 5000RPM with the lights on high beam and measuring the voltage across the battery terminals. The manual rather cryptically suggests that the charging voltage should be more than the battery voltage “at rest” and less than 15.5 volts. Given that it was slightly lower than before commencing the test, it seemed a fairly safe bet that the bike was no longer charging the battery.

Back in the day, “they” used to say that Hondas were notorious for cooking regulator/rectifiers. My first Honda (the mighty Super-blackbird – the bike that was ever so briefly the fastest production model motorcycle on the planet) certainly managed to break this component and overcharge the battery in the process. It appears that Honda beefed up this component as my next Honda (a 929 Fireblade) burnt out the stator coil. It was looking like the VFR had suffered a similar fate.

The workshop manual specifies various tests – measuring resistance and testing continuity of various connections to determine the faulty part in the charging circuit. On the right hand side of the motorbike is the connector from the stator into the charging circuit. It is described as being a “3P natural connector” although “white” seems to be an equally suitable term…

Untitled

According to the manual, there should be no continuity between any of the three yellow wires (in the plug) and ground. The multimeter revealed that two of the three wires did indeed have continuity to ground and hence I had found the problem! As for what to do about it, well that is a story for another time.

Engines – Part Two.

Previously I described an engine as having pistons that travel up and down inside a cylinder.  The piston is attached to the engine’s crankshaft via a conrod.  Each piston in a four stroke engine has four distinct phases through which it travels.  For two of these “strokes”, the piston travels downwards. The other two strokes, the piston travels upwards.

1 - Intake Stroke The first downward stroke is referred to as the Intake stroke.  At the top of the cylinder head, there are valves that open and close at different points in time.  The inlet valve (or valves) open during the intake stoke, allowing a fuel and air mixture to enter the cylinder.  Because the piston seals the cylinder when it travels downwards, it creates an area of low pressure above it.  This helps draw the fuel-air mixture in.
2 - Intake Stroke completed At around the time the piston reaches bottom dead centre (BDC) the inlet valve closes, sealing the gasses in the cylinder. The piston then starts its second stroke: the compression stroke.  The piston travels back up, compressing the fuel-air mixture at the top of the cylinder.  The difference in volume between when the piston is at the bottom of its stroke and the top of its stroke, is commonly referred to as the engine’s compression ratio.
3 - Compression Stroke Compressing a flammable gas in the presence of oxygen is somewhat fraught with problems.  Compressed gas gets hot.  Hot flammable gas can combust!  The octane rating of a fuel denotes how stable it is.  Engines with high compression ratios need high-octane fuel to prevent uncontrolled detonation during the compression stroke.
4 - Compression Stroke completed Once the piston reaches top dead centre (TDC) the compressed fuel-air mixture is ignited by the spark plug. This causes the gas to ignite and expand rapidly.
6 - Power Stroke The valves in the cylinder head remain closed at this stage, so the only way the gas can expand is by forcing the piston back down the cylinder. This is referred to as the Power stroke.
7 - Power Stroke completed All things going well, by the time the cylinder reaches BDC again, all the gasses have been burnt.
8 - Exhaust Stroke No further kinetic energy is to be gained from them and they need to be removed from the cylinder, ready for the next cycle. The outlet valve (or valves) open and the now rising piston forces the exhaust gasses out through them. This is known as the exhaust stroke. At the completion of the exhaust stroke, the exhaust valve has closed and we are ready to repeat the entire process.

Two stroke engines work in a similar manner, but combine the intake /power strokes together and the compression / exhaust strokes.  How this is done, is a story for another time.

Engines – Part one

Be they two-stroke, four-stroke, diesel or rotary, internal combustion engines all work on the same principle:  Fuel and air can be made to combust.  Doing so causes rapid expansion of the resulting gasses.  This expansion of gasses is “mechanically contained” in a combustion chamber in such a way that the energy from it is used to turn a crankshaft.

Rotary engines are dissimilar enough to require their own discussion, so if these are of interest to you, then I recommend you look elsewhere.  The other types of engines mentioned, all feature a piston attached to the crankshaft via a conrod travelling up and down inside a cylinder.

When the fuel-air mixture is ignited the piston is at the top of its stroke and the gasses expand, forcing the piston down.  This is the “power stroke”.   The distance the piston travels from the top of its stroke to the bottom, is measured and referred to (rather unimaginatively)  as the engine’s “stroke”.  The diameter of the cylinder in which the piston travels is measured and called the “bore” size.

Together, the bore and stroke of an engine are used to define the engine capacity.  The volume of the cylinder , which has the diameter of the bore, and the height of the stroke, gives you the capacity of a single cylinder.  Multiply this figure by the number of cylinders in the engine and you have the total engine capacity.

EXAMPLE:
If a four cylinder engine has a bore of 74mm and a stroke of 58mm…
For those of you who do not remember:
V = π * r ² * height.
The radius is 3.7cm (half the bore) and the height is 5.8cm.
Using the formula, a single cylinder, has a capacity of 249.45 cubic centimetres (cc).
Multiply this by the number of cylinders and you end up with a 997.8cc engine.

As a piston travels up and down inside a cylinder, it must come to a complete stop and change direction.  Arresting and reversing the piston’s momentum takes energy.  The faster the piston is travelling, and the heavier the piston is, the more energy is required.  This energy is supplied by the momentum of the crankshaft.  This means that some of the engine’s output is spent  in changing the velocity of the piston.  For this reason, there tends to be an optimum speed at which the piston travels.  The longer the stroke of an engine, the further the piston travels.  So, in an engine with a long stroke, the piston travels faster at a given crankshaft speed than in an engine with a short stroke.  As a rule of thumb:  the longer the stroke of an engine, the slower the maximum revolutions per minute (RPM).

There is no ideal size for an engine bore or stroke.  A lot depends on what the engine is powering.   Engines that need to move a lot of weight, require more torque.

Torque is a twisting force applied to an object, like a wheel or a crankshaft.   For our purposes, we will consider that torque is measured in pounds-force feet (lbf-ft) meaning the equivalent of a given force, in pounds, acting on the end of a lever of length in feet. … For example, standing with 180 pounds body weight on a lug wrench one foot long yields 180 lbf-ft of torque.

Work is the application of force over a distance. Unfortunately, the units used are the same (pounds times feet) but we write this as ft-lb just to distinguish it. The real difference is that in this case, the “feet” part means feet of movement.

Power is the application of work within a finite time. 550 ft-lb of work in one second is one horsepower.

From this, we can see that torque and power are related.  In fact:

hp = (torque * RPM) / 5250
(Where torque is measured in lbf-ft)

Another rule of thumb is that: engines with a longer stroke produce more torque.  There are other factors that influence power and torque outputs of an engine.  Cubic capacity never goes astray when you need more power and torque…

A motorcycle does not need to shift a vast amount of mass – but tends to be size constrained.  (unless of course, you are prepared to just jam in a big car engine)

More weight conscious sports bikes tend to concentrate on obtaining a high power figure, over obtaining a high torque figure.  Given the formula above, we can see that to produce more power, we either need more torque or more engine revs.  So, sports bikes tend to concentrate on producing power by spinning quickly, which means they end up having a short stroke.

Getting the fuel-air mixture into the combustion chamber also affects power and torque figures of an engine, but that is a story for another time.