4RT Wasted spark ignition or not?

[aussiechris]

Warning: long message with copious amounts of techno-babble.

I was reading an old discussion on this forum about whether the 4RT runs a “wasted spark” ignition (that is one spark per revolution of the crankshaft) rather than a more conventional (four stroke) one spark every 2nd revolution.  It was also suggested that the fuel injection is not sequential but rather injects fuel continuously.  Certainly Honda are not averse to using wasted spark ignitions - many of their multi-cylinder bikes do just that.  And given the engine does NOT have a cam position sensor, both the above would be reasonable assumptions.

However, I think I have managed to prove that the engine does only fire the spark plug every second revolution and the fuel injection also occurs every second revolution timed to intake valve opening (which makes sense given the location of the injector).

How?  Let’s get technical…

I made up some test harnesses to enable me to intercept the wiring at the trigger (pulse generator) coil, the ignition coil, and the fuel injector so that I could view what’s happening on an oscilloscope.

 

At my (slow) idle speed of around 1700rpm, the crankshaft is rotating 28.3 times per second or once every 35 milliseconds.   The crank trigger coil sends pulses to the ECU every time a hole in the flywheel passes by.  The flywheel actually has a row of holes/slots that pass the trigger coil, then a gap with no holes and that gap or burst occurs every 35mS.  The pic below shows the output of the trigger coil on Ch1 in yellow.

The blue trace (Ch2) is the ECU signal going to the ignition coil primary.  You will note that it only occurs every 2nd burst of trigger pulses.  The ignition coil pulses are about 70mS apart (each grid square being 10mS) so that is about 14.2 times a second or 850rpm.  That’s half the crank rpm of 1700.

So what about the fuel injection?  Unfortunately I only have a two channel scope so in the following image, the Ch1 yellow trace is now the fuel injector signal.  The blue trace is still the ignition coil showing the relationship between the two.

 

You can see how the fuel injector fires in between each ignition pulse.  This is always the case regardless of rpm.  The injector is firing every 70mS just like the ignition coil except one crank rotation out of phase.  I did some measuring and the injection pulse* occurs 34.8mS before ignition spark…

Given the time for two revolutions (720°) is70ms, then fuel injection is commencing about 358° before the spark which sounds about right - just before the intake valves open at the end of the exhaust stroke (taking into account spark advance and valve overlap).  The injector pulse duration (the negative bit) at idle is about 2ms long (just over 20° of crank rotation) but is visibly longer when the throttle is opened.

 

So, in summary, this engine definitely fires the spark plug every second revolution (at the end of the compression stroke) and injects fuel sequentially just prior to the intake stroke.

 

That just leaves one question for ya’ll to ponder…  Since the engine does not have cam position sensor (or points), how does it know which stroke it is on?  It would be easy to suggest it just fires the spark plug every second crank trigger but it has to pick the correct one to align with the camshaft otherwise it wouldn’t run.  I have a theory on how it might achieve this every start but wondered if someone else would like to offer an opinion.  Looking forward to your comments.

 

* Note: the above traces are referenced to the +13.5v power.  Both the ignition coil and fuel injector are switched to ground by the ECU.  The spark plug fires when that ground is removed (the end of the negative pulse) but the fuel injector opens when the ground is applied (start of the negative pulse).

Edited 6 hours ago by aussiechris

[konrad]

17 minutes ago, aussiechris said:

 Since the engine does not have cam position sensor (or points), how does it know which stroke it is on? 

Instantaneous crankshaft speed.  The crank slows on compression and speeds up on expansion.

Almost certain it must fire every crank revolution until it's actually "running", though.

Nice to see another curious person on this forum!

 

[aussiechris]

6 minutes ago, konrad said:

Instantaneous crankshaft speed.  The crank slows on compression and speeds up on expansion.

Almost certain it must fire every crank revolution until it's actually "running", though.

Nice to see another curious person on this forum!

 

That's certainly a possibility Konrad.  I know my six cylinder car does that so it can detect a misfire and log a code indicating which individual cylinder is missing.  It could explain why there's a row of trigger holes in the flywheel rather than a single slot like other bikes.

But I was thinking of another possibility.

And yes, too curious for my own good!

[lemur]

Rule of thumb to determine if a spark is wasted or not is to look at where the timing in taken from.  Crankshafts rotate at twice the speed of the valve camshaft so if it is ignition timed off the crank logic would dictate a wasted spark and if it is timed off the camshaft rotation that will happen half as often.  

Edited 5 hours ago by lemur

[sportsawyer]

2 hours ago, aussiechris said:

... That just leaves one question for ya’ll to ponder…  Since the engine does not have cam position sensor (or points), how does it know which stroke it is on?  It would be easy to suggest it just fires the spark plug every second crank trigger but it has to pick the correct one to align with the camshaft otherwise it wouldn’t run.  I have a theory on how it might achieve this every start but wondered if someone else would like to offer an opinion.  Looking forward to your comments. ...

I the past, I suspected the ECU was looking at MAP vacuum to determine the intake stroke and know which crank revolution it was on.

[lemur]

I would want to think the part called the 'ignition pulse generator' has something to do with ignition pulse generation. 

[lemur]

1 hour ago, sportsawyer said:

I the past, I suspected the ECU was looking at MAP vacuum to determine the intake stroke and know which crank revolution it was on.

If they do it that way the vacuum sensor must be incorporated into the 4RT throttle body.  Wasted spark is not a problem wasted fuel would be a problem, only makes sense they would need to inject only when an engine vacuum is present 👍