PBR TOP FUEL BIKE

          Engine Vibration

The crankshafts are nearly finished and I've been fretting a bit about crankshaft balancing and engine vibration.

 

The symmetrical layout of an in line 3 cylinder design with 120 degree crank pin indexing results in perfect balancing of the primary and secondary forces. If the crank itself is balanced and the weight of the piston/rod/pin assemblies is identical, the whole assembly will be rotationally balanced.

 

Unfortunately, this layout induces primary and secondary “rocking” moments along the crankshaft, evidenced as end to end crankshaft vibration. The majority of in line 3 cylinder production engines use balancer shafts to counter-balance these forces.

 

The magnitude of the “rocking” moments are proportional to the square of engine speed and are linearly dependent on the piston weight, cylinder spacing (bore centres) and crank throw (½ stroke). Con rod length has an influence on the secondary forces.

 

At 9,000rpm the motor for the fuel bike will have a primary moment of around 15000Nm, and a secondary moment of around 3400Nm. I have absolutely no idea what this means in practical terms, but it sounds a lot!

 

To try and get a feel for the level of vibration that might be experienced I calculated the primary and secondary “rocking” moments for my old 975cc Triumph Trident (in line 3 cylinder, 120 degree crank angle, no balancer shaft). The values come out at 4760Nm and  1330Nm.

 

The main cause of the higher values for the new motor is the piston weight (around 3 times heavier than the Trident motor). (Note: I have only used the piston & pins weights, with no allowance for the con rods – as the con rods for both engines weigh about the same, if some allowance for the reciprocating mass of the con rod was included the ratio of the calculated forces would be lower).

 

So, I might expect the vibration on the new racebike motor to be around three times more than the old Triumph. The 975cc Trident was a little bit lumpy, but nothing particularly bad – I rode the bike for 1000s of miles and had no great issues with comfort or vibration associated mechanical problems. The bottom line is that the new motor is certainly going to jump around a bit, but taking account of the short duration of operation, it should be tolerable – although I may be investing in a pair of soft handlebar grips!

 

 

The Ignition Solution 

 

What to do about the ignition?

 

The cylinder head is configured for twin plugs; there is room to fit a third, central plug if desired in the future (although maybe not a third magneto).

 

Comparing ignition systems based on manufacturer's technical data is not straightforward - the ignition equipment vendors tend to state the performance of their equipment on different (and unspecified) assumptions, so its really difficult to compare on a like for like basis.

 

There is a wide range of ignition equipment available, ranging from a simple points/coils arrangement to a ProMag44. With a 4 banger its pretty simple. Pay your money and bolt on one or two ProMag44s and away you go. Not so straightforward with a triple.

 

Lets assume that all the ignition systems available can produce sufficient voltage to provide a spark at the plug – they would not be much use otherwise.

 

All combustible substances have a minimum ignition energy (MIE); this is the lowest energy spark that will ignite the combustible/air mixture under “most advantageous” conditions – that is, the absolute lowest value possible. The MIE of most hydrocarbon fuels is in the range 0.2 to 0.3mJ; for example the MIE of unleaded gasoline is around 0.29mJ. I haven't been able to find a definitive MIE for nitro, but I have no reason to believe it would be significantly different to this normal range. Lets put this into perspective, that spark from your finger to earth after you've been dragging your feet across the carpet can have an energy of up to around 50mJ. So its easy, right?

 

In the real world, and particularly in reciprocating internal combustion engines running supercharged on nitro, things don't get anywhere close to ideal. Additionally, as the mixture moves from stoichiometric towards the inflammability limits the MIE increases rapidly. So, in practice, the MIE requirement is much higher – and if you don't have a high enough energy spark the mixture doesn't ignite; it doesn't matter how long the spark lasts or how many you have. It follows that the higher energy spark available, the more difficult the mixture that can be fired.

Once you have enough spark energy the total power becomes important – longer spark duration produces more spark power and more spark power translates into a larger flame front to propagate the flame through the mixture (it burns better).

 

Although an electronic ignition/coil (CDI) type ignition would be much easier to engineer, they just don't provide enough spark energy for blown nitro applications – which most likely explains their absence.

 

Electronic ignition systems are available that combine capacitive discharge (CD) and inductive elements (such as the MSD Nitro Harley system) which seems to work OK on the “Harleys”. Problem is, for my application I would certainly require 3 complete systems (and maybe six!).

 

If electronic ignition is eliminated (on capability, complexity and/or cost grounds) the only available option is magneto ignition.

 

Until recently Vertex manufactured a 6 cylinder magneto; new items are still available from parts suppliers. The stock vertex magneto is around 1.5A; this can be uprated to around 6A in combination with an external coil. Just not big enough, and I don't really want to start with “obsolete” equipment.

 

This leaves SuperMags or ProMags, neither of which are available in 3 or 6 cylinder versions.

 

The solution is to run 4 cylinder magnetos at 3/8 engine speed – this produces a spark every 120 degrees to suit the 3 cylinder configuration. The coils provide the HT to 6 cylinder distributors (only using 3 of the plug lead terminals) fitted to the end of each camshaft (running at half engine speed). Not as neat and tidy as a magneto mounted distributor, but it should do the job.

 

In the end I elected to go for SuperMag SM+ magnetos and “monster” black nitro coils supplied by FIE. This about the most powerful points magneto set up available. I recognise the drawbacks of points type magnetos compared with their electronic counterparts and the decision was really based on the following:

This is a bit of an oddball application and I needed an ignition supplier who would work with me. Spud Miller at FIE has been way beyond the call of duty in this respect.

ProMags only come in 8 cylinder form, so I would end up having to “waste” a lot of sparks.

ProMags are significantly larger than SuperMags. Also need to find space to fit points boxes, etc.

I need to be able to start the bike. We've always found it more difficult to start since we changed to a ProMag on The Dealer, particularly when running on the crank sensor.

 

What about the acknowledged issues with points type mags?

 

Points “flutter” and “bounce”. The magneto generators are designed for and tested at 10,000rpm engine speed. Running the mags at 3/8 engine speed (rather than the normal ½) provides a bit more margin.

 

Points burn out and wear. The mags aren't intended for high mileage use. The ignition is fitted with all the components designed for the duty, but it is inevitable that they will require more looking after than an electronic mag (for which you just keep spares). As I don't have any distributor on the magneto generators it would be fairly straightforward to engineering a points cooling system using CO2.

 

Imprecise ignition timing. Every points mag I have ever come across has had some deviation in the precise points opening on the different points cam lobes – typically this is in the range +/-2 degrees, but I have seen +/-4 degrees. There are several contributory factors: Machining tolerances on magneto components, wear, magneto drive factors, ignition timing measurement uncertainty limits. Running twin mags in the past I have always tried to get the same “average” ignition advance on each cylinder.

With the proposed magneto generator drive arrangement:

On ignition event 1 magneto cam lobe 1 fires cylinder 1

On ignition event 2 magneto cam lobe 2 fires cylinder 2

On ignition event 3 magneto cam lobe 3 fires cylinder 3

On ignition event 4 magneto cam lobe 4 fires cylinder 1

On ignition event 5 magneto cam lobe 1 fires cylinder 2

and so on…

So any differences in ignition advance between the individual magneto points cam lobes are cycled across the cylinders, rather than one cylinder always being advanced or retarded. I've convinced myself that this is a good thing.

The other thing factor than can affect the precision of the ignition timing is the spark trigger mechanism. The MSD ProMag web-site suggests that their magneto trigger can give rise to deviations in ignition timing of around +/-2 degrees, whereas the crank trigger only +/-0.5 degrees. The quality of the mag trigger will have some impact. Smaller deviations are obviously better; however, we've run The Dealer on the ProMag44 mag trigger and crank trigger “back to back” and not seen any difference in performance (et & mph) outside normal deviations.

 

Ignition timing adjustment. This where the electronic magneto really has the edge, being able to do all sorts of trickery with the ignition advance curve. I will be using much cruder pneumatically actuated equipment – but I believe this will be adequate, in combination with the far greater degree of clutch control the air clutch provides.

 

Spark energy and power. The SM+ magnetos can't compete with a ProMag 44; In MSD speak the SM+ mags are equivalent to a “ProMag30”. With the proviso regarding ignition performance data provided by the vendors, the spark energy from the SM+ mags I have is 730-740mJ (measured), compared to 800mJ quoted by MSD for a ProMag 44. MSD claim a spark duration of 26 degrees, compared with 22 degrees for the SM+.

As far as I am aware no fuel bike has run sub 6 seconds without the use of at least one ProMag 44; so another challenge for the new bike.

 

Finally, we've got to be able to start the new bike. I reckon I will need to spin the motor at around 425rpm for starting (another implication of running the magneto generators slower) – so the facility to start off the clutch drive pulley will be included.