Friday, 19 August 2016

Idle Control Valve & More Wiring

Idle Control Valve

One of the things that was bugging me about the whole install was the fact that because I had converted the carburetors to throttle bodies and disabled the choke, cold starting was still a bit of an issue. There was a fine line between setting the throttle stop such that the engine would run when cold and still not idle massively high. I had got it to a point where it would idle at c.2,000 rpm when hot but when cold it would struggle to life & I couldn’t touch the throttle until a reasonable amount of heat had built up in the engine. It was OK for the short term but it wasn’t good enough for a road engine so I started looking into getting some form of idle control implemented.

As most idle control valves are based on a stepper motor driving open the throttles when cold I began to look at these. I contemplated mounting a stepper motor to the throttle bodies to drive the throttle stop screw in and out so that I could vary the closed position depending on coolant temperature. The issues with this were the placement of the stepper motor & also the stepper motor drive. The Microsquirt does not have a stepper motor output so I would need an external unit to convert the PWM output of the Microsquirt into something that could be fed into a stepper motor. Space constraints won out in the end. The thermostat housing is located pretty much exactly there the motor would need to be mounted and I had already moved the thermostat housing as much as possible just to get the fuel rail to fit.

I also considered a mechanical system with a thermostat that would retract a plunger as coolant temp rose and close the throttle more but the same space constraints also killed that idea.

I then thought that I could bleed air past the butterflies by using the carb sync nipples that I was already using to measure MAP from. Having air leaking in here results in rough running on a carbureted system but with the FI system I would have the ability to add fuel to compensate and make use of the additional air being drawn in. A solenoid valve could be driven by PWM directly from the Microsquirt to regulate the flow of air past the butterflies and the MAP sensor could be T’d in between the solenoid & the split to all the ports so that MAP could still be sampled in the same way as before.

After a little research I purchased a Bosch canister purge valve used in many cars and a selection of silicone hose & T pieces to plumb in the system. The diagram below shows a schematic of the plumbing I ended up with.

The idle control valve so far has only been tested for a very short time on the bike but so far it is looking very promising. When the engine is warm, the difference in engine speed between valve closed & valve open is 1,000 rpm and I can also use the valve to control to a target idle speed in closed loop mode. It certainly needs more running to dial in the settings but it is a big step in the right direction.

Wiring Harness Issue 02

Given the addition of an idle air valve, I needed to run the signal wire from the ECU to the injector harness to drive the valve so that required a small change to the ECU harness. Small changes spiral quickly with me though and this was no exception.

Even though I had spent some time this time last year tidying up the wiring harness, I had become pretty unhappy with it as time went on. In diagnosing a faulty main relay some months back I became sceptical about the quality of connections I had made in the harness. Since most of these connections were made at the very beginning of the project when I had little experience and were added on a “when needed” and “where is accessible” basis, I did not have a clear idea about how reliable they were and where the connections were physically located within the harness. The harness was also far bulkier than I cared for and was generally not up to my current standards.

At the time I started to doubt the harness I had decided that I would build a completely new main bike harness incorporating the ECU harness and dash and a few other upgrades that would make the bike a bit more modern so I thought the old ECU harness would do the job until I got the main harness built.

However, having to run a single wire along the harness presented the perfect opportunity to tear it all apart and start again to make the ECU harness tidier and more reliable until the upgraded bike harness would be ready to drop in. The main aims for the new ECU harness were:

  1. Remove all unused wires from the harness. Previously I had just chopped the unused wires back but they still ran within the harness up until the front of the pillion seat.
  2. Replace any wires within the harness which were deemed overkill for the level of current they are expected to carry.
  3. Replace all previously spliced wires with new more reliable spliced connections.
  4. Tidy up the power connections around the fuse box & add proper relay connectors to make removing and replacing components more foolproof & straightforward
  5. Tidy up how the external coil driver box interfaced with the harness
  6. Mainly use the process to create a complete & up to date harness drawing and pinout chart including connector specs, wire routing, wire sizes, splices & splice locations and harness lengths. All this would take more time now but would really help with any fault diagnosis and harness alterations in the future. It would also get me one step closer to a full set of documentation to capture all the alterations & modifications I have made to the bike throughout this project.

While drawing up the new ECU harness I decided to change the way injector power was being delivered to enable me to fit in the idle valve wiring in the same connector that was already in the harness. Previously, each injector bank (2off) had its own fused power supply so I merged these two supplies into one which also provided power to the idle valve and allowed me to use a 4-way connector between the ECU & Injector harness. That change did however force a rebuild of the injector harness even though I was reasonably happy with that. At the same time it was a good excuse to make a few nice changes and make it altogether more robust.

First was to remove all the wires from the ECU Ampseal connector that were not being used. Also any signal wires which had been cut and spliced previously were removed and replaced with fresh wire. The harness was then laid out and laced together with the branches coming off the main trunk at the correct locations. Then connectors were added and the whole lot wrapped. Installed on the bike it is now far easier to install & remove the harness and it looks much tidier also.

Laced ECU Harness

Installed ECU Harness
While that all looked good, I felt that wrapping the harness tended to twist the bundle a little when wrapping and the wrap was also a little untidy around the areas where the branches broke away from the main bundle so for the Injector harness I took a slightly different approach. I sleeved the wire bundles with DR-25 and then covered the joints with adhesive lined heatshrink to provide some rigidity. I also redesigned the harness so that the main bundle would be tied to the throttle bodies just below the airbox rather than onto the fuel rail where it was previously.

Completed Injector Harness

Installed Injector Harness

Installed Idle Valve

Thursday, 18 August 2016

Back To Basics

I realise it is close to a year since I updated this but better late than never. Not much has happened in the way of real engine calibration since the last update but still some reasonable progress in terms of fine tuning settings. The bike got MOT’d right after the last blog entry and had a few outings afterwards but hasn’t really moved since Christmas.

I wanted to check a few things that were bothering me about the Microsquirt control and so ended up carrying out quite a lot of bench testing on the ignition side of things over several months.

I actually don’t remember where it all started but at some point during the last year I started to question the accuracy of ignition timing provided by the Microsquirt ECU. I carried out some tests using a crude trigger wheel simulator on the bench & then on a running engine which confirmed my suspicions; ignition timing appeared to drift as much as 5 degrees at 18,000rpm. On top of this I noticed two further uncertainties while testing.

  1. Battery voltage as read by the ECU did not track actual battery voltage. There appeared to be some hysteresis in the measurement as the ECU recorded a different offset depending on whether the battery voltage was rising or falling. This was obviously an issue as it would result in incorrect coil dwell times & injector dead times.
  2. During ignition coil testing using the ECU’s output test mode, coil dwell times measured did not match what was demanded by the ECU even with battery voltage compensation disabled.

When I first noticed the timing accuracy issue I figured I could simply get a new, higher resolution trigger wheel made to replace the standard 12-3 wheel. However I hadn’t reckoned with the fact that the Microsquirt doesn’t have the processing power to cope with a higher resolution wheel at the kind of engine speeds seen on the CBR250. The Megasquirt developers have stated that even with the low resolution wheel on the CBR250, 18,000rpm is pushing the boundaries of the Microsquirt hardware capability and that an MS3Pro ECU would do a far better job at controlling the engine at these speeds. Coming out of this, I will most likely be upgrading to an MS3Pro ECU sometime in the future but given the expense and the fact the wiring harness would need to be redesigned to work with the new ECU I tried to find a way to make the Microsquirt work acceptably until I was ready for the upgrade and so I continued with reasonably extensive bench testing in order to measure and understand how the ECU controlled the ignition side of things.

Ignition Timing Accuracy
Firstly I wanted to spend some more time bench testing the ignition control both to understand the issues better and to be able to present the issue properly to the Megasquirt developers which would help find out if the MS3Pro would actually fix the issues. That meant making sure that all other variables were as accurate as I could make them. The most important of these was the trigger wheel offset relative to TDC. I had determined this value early in the project using a timing light so it was potentially not as accurate as it could be.

To determine this properly with the oscilloscope, I needed to compare it directly with the output of the standard MC22 ECU at a known engine speed and spark advance. I wanted to have a way of accurately simulating the crank trigger signal as would be seen by the ECU in a running engine. There are lots of ways available to simulate hall effect trigger signals but I specifically wanted to simulate the VR type of signal that creates a zero crossing. After some research & trial & error I settled on using an Arduino board to create the signals using a modified version of the Ardustim code.

The Ardustim code normally generates a hall type signal by asking a single pin to go high at specific times. With some modification to the Ardustim code I ended up creating a VR type signal by switching two separate pins & merging them into a single channel. The voltage output from the merged channel would depend on the combined state of the two Arduino pins.

Both pins high = 5V; Both pins low = 0V; 1 pin high + 1 pin low = 2.5V.

The output is fed through a transformer to create the needed zero voltage crossing. The simulated VR signal could then be used to determine the 1st tooth angle offset to TDC from the standard MC22 ECU.

As I had a CBR250RR(R) ECU, the benchmark from the manual is spark demand at 20°BTDC at 1,500rpm. Measuring the time delay on the oscilloscope between the 1st tooth trigger & coil firing signal and converting to degrees at 1,500rpm gave me a 1st tooth angle of 70.5°BTDC. 0.5° closer than I had measured with the timing light.

1st Tooth Offset Determination. Note The Simulated VR Signal
With the foundation settings correct in the Microsquirt I then proceeded to map the ignition angle error across the engine speed range to build up some data and understand what the exact nature of the error was. It was only when I actually wrote the error values down that I discovered that the errors were consistent in terms of time throughout the engine speed range. This led me to wonder if this was something that could simply be corrected for in the calibration by adjusting the spark hardware latency offset setting. I had not used this previously as I had understood that it was to be used to correct for any delays which might exist within external hardware and since the errors I had been seeing were from within the Microsquirt itself, I ignored this feature.

I made some small adjustments to the latency offset and rechecked ignition timing across the speed range each time until I was happy that the measured ignition timing was consistent with what was being demanded.

Voltage Reading
It was actually quite straightforward to fix the battery voltage reading error within the calibration. I used a digital benchtop power supply to vary the voltage supply to the ECU & noted the voltage readings within the ECU. I quickly found that the different voltage readings depending on if the voltage was increasing or decreasing was being caused by the smoothing function which I had set quite high initially. Once the smoothing was removed, the ECU voltage readings tracked consistently all the time but were offset slightly from the supplied voltage.
I used the measured error to determine new values for voltage at max & min ADC to change the voltage reading calibration which brought the voltage readings in line with the supplied voltage.

Coil Dwell Time
While testing and characterising the ignition coils using the controller’s output test mode to drive the coils, I found that the measured coil dwell tended to be longer than what was being demanded from the ECU by a factor of c.1.14. This did became a little worry at the time but I have since noticed that the error appears only when controlling the coils in the output test mode. When the engine is running (or simulated) the measured dwell time matches what is being demanded by the controller and so this became a non-issue.

Ignition Coil Characterisation
A major unknown in the ignition system was the dwell time required for the ignition coils.

First, the standard MC22 RR(R) ECU was mapped in order to determine what dwell times it was requesting at different engine speeds and then compared against how the Microsquirt code handles dwell.
Looking at the dwell times that were being requested by the standard ECU and how they changed as engine speed increases gave the impression that the coils may be performance limited at high speed as the dwell time at 17,000rpm is significantly shorter than at 5,000rpm for example and if the dwell being provided at lower engine speeds is necessary for good coil performance then it would mean that the coil is not getting fully charged at higher engine speeds and ignition performance may be compromised.

This observation prompted an investigation into trying to fit some higher performing pencil coils from a more modern machine to try and get consistent ignition performance throughout the speed range. The biggest issue was and still is finding coils which will physically fit in the MC22 plug bores.

All TCI coils found are too high and protrude quite a lot above the cam cover and foul the radiator fan.

CDI coils such as installed in ’97-‘99 Suzuki GSXR600/750s & 2009 Yamaha YZF250 seem to be the smallest coils physically and could be made work but even these are c.10mm longer than ideal for the MC22. The issue with the CDI coils then is that while they can be made work with a TCI ignition system, the charging current is far higher and the dwell times are far shorter than a normal TCI coil and the ECU and coil drivers would need to be capable of controlling them very accurately.
The charging current can be dealt with using an appropriate ignition IGBT specced for the high current & fly-back voltage. I did purchase a number of appropriate transistors which were capable of driving the CDI coils successfully during bench testing.

However it was found during testing that the Microsquirt firmware did not have the dwell demand resolution needed to accurately control the GSXR CDI coils. The Megasquirt firmware can only control dwell to the nearest 0.1ms but the GSXR coil nominal dwell is c.0.3ms so accurate control of the dwell is impossible with the Microsquirt controller. Due to this control limitation, the coil upgrade idea has been parked for the foreseeable future.

Given no alternative ignition coil solution was on the horizon, the standard MC22 wasted spark coils were tested to characterise them and ensure they were being driven properly by the ECU. The coils are nothing special for the MC22 as one might expect for an engine that was being asked to rev higher than most other engines at the time. They are TEC MP08 coils which seem to have been the standard Honda wasted spark coil of choice during the 1990s.

A current clamp was connected to the oscilloscope to observe how the charging current increased with dwell time & determine the optimum dwell time for a range of battery voltages to provide consistent performance. Observing the way that current draw increased with time it was decided to limit the charging current to 2.75A. The standard ECU does not correct for battery voltage and dwells the coils for 6ms at idle and up to 4,000rpm. As can be seen in the current trace below, this level of dwell overcharges the coil and dwell times could be significantly reduced without too much of an effect on ignition performance. Reducing dwell times would also ensure the coils are performing consistently across as wide an engine speed range as possible. At 12V, the dwell time to 2.75A is 2.5ms and even at 10V there is no sense in dwelling the coils for more than 4ms although it does not reach 2.75A.

TEC MP08 Primary Current at 12V Supply, 6ms dwell
This investigation showed that as long as battery voltage is kept above 12V there would be no reduction in ignition performance across the full engine speed range. Another benefit is that by keeping the dwell as low as possible at lower engine speeds, this would reduce the load on the charging system compared to the standard settings. Also the old adage, “leave well enough alone” holds true as there is no point in chasing the COP coils option when there is nothing suitable on the market at the moment for reasonable money. For a bigger engine it would be worth it purely to free up some space and provide the option for sequential ignition at a later date.

While getting all this information took a lot of time and investment in new equipment, it is worth it to ensure that the basics of the system are working correctly otherwise it would make future calibration more difficult.