Fueling without cam sensors

[LakeWylieJoe_imp]

This is a somewhat down-in-the-weeds technical question that I have not seen addressed before.

I just finished my third valve clearance check and adjustment at 24,000mi on the odometer. A few valves were at the upper or lower spec limit each time I've inspected them and I always set them back to the center of the spec range. It seems foolish to me to leave any at either extreme end of the range because if they drift at all in that direction, then it's out of spec.

Anyway, while reinstalling the cams, a question occurred to me: How does the ECM know if a particular cylinder is TDC on the compression stroke or TDC on the exhaust stroke? Unlike most modern engines, the CB1100 does not have cam position sensors; it relies completely in the crank position sensor(CKP). The CKP can only give information that a cylinder is TDC, but not whether it's compression stroke or exhaust, which is 100% dependent on the cam positions (which turn at exactly 1/2 the rate of the crank).

The ignition system doesn't need to know the difference as it sparks the #1 & #4 cylinders together and the #2 & #3 cylinders together. A spark is wasted during each firing because only one of the two cylinders associated with each coil fires is needed for a given crank cycle. A wasted spark is immaterial, though.

However for the fuel injection, you can't fire a fuel charge into a the back of a closed valve then again into an open port on the next crank cycle, so how does the ECM know when to fire the injectors? The system must determine if a particular cylinder is at TDC compression or exhaust somehow.

On the single cylinder CRF dirt bike (which also has no cam sensor), the CKP can sense the slightly-slowing crank rotation during the compression stroke and the increasing rate on the power stroke to determine when TDC compression is. That wont work on an inline 4-cylinder, though, with a power stroke every 180 degrees of crank rotation.

Anybody of you smart guys have any thoughts on how Honda overcomes not having cam sensors with the CB1100 motor?

[pekingduck]

That's a good question. On a single, it could take a MAP sensor reading to sense intake, but on our CB1100, the MAP sensor reads the average from the airbox.

Most new Hondas don't use a cam sensor (except new GL1800)

[peterbaron]

CrPS is a magnetic or Hall effect sensor that generates voltage with cooperation of a tone wheel. Missing tooth from a tone wheel, ring gear or flywheel is TDC. These two tell ECM the exact position of the piston. Crank to cam = 2:1 = 720*:360 = one full cycle of four stroke engine.
ECM uses its program to ground out particular injector, while B+ supplies constant power when IGN is on.


Tone wheel with missing tooth/TDC

This missing tooth/TDC will make a different kind of pulse as seen in pic below


CrPS pattern with TDC signal

CrPS + missing tooth + ECM do the trick, this is how I see it.
Not sure if CB uses tone wheel, ring gear, flywheel or...?? as a TDC reference, but must use something.

[pekingduck]

CB1100 only has a crank pulse sensor on the end of the crank (with "missing teeth") that determine crank/piston position. Ignition and fuel mapping can then be determined by the ECM with input from the other sensors

But since the camshafts are spinning half of crank speed, how is it determined which if the two crank rotations the ECM supplies fuel on? To paraphrase LakeWylieJoe, there can be a wasted spark, but not a wasted squirt. So how is that determined?

[peterbaron]

To my best automotive knowledge, all it cares is to see TDC of cyl #1, then ECM program will take care of the rest, knowing its firing order = 1,2,4,3...
...when I am riding my CB, I do not care how it works....I'll fix her if she troubles, IF she does

[pekingduck]

The latest Honda CBR1000RR uses a cam pulse generator, as does the latest Honda Fit. But the NC700/750X twins, which base their engines on the Fit motor, do not have one.

[peterbaron]

Honda engineers always the best and surprising

[Ben70_imp]

If anyone has a multi channel oscilloscope available, we could hook into cyls 1 and 2 injectors and determine if they are firing at the same time. If so, it would be ported fuel injection with a waste pulse, or possibly continuous pulse like the old throttle body injection on cars.
If they don't fire at the same time, it would be sequential fuel injection.
I don't remember reading anywhere that it specified sequential fuel injection.
Interesting point to determine, in any case.

Ben

[the Ferret]

Like reading Kevin Cameron lol

[pekingduck]

If anyone has a multi channel oscilloscope available, we could hook into cyls 1 and 2 injectors and determine if they are firing at the same time. If so, it would be ported fuel injection with a waste pulse, or possibly continuous pulse like the old throttle body injection on cars.
If they don't fire at the same time, it would be sequential fuel injection.
I don't remember reading anywhere that it specified sequential fuel injection.
Interesting point to determine, in any case.

Ben

Throttle body injection worked best in multi-cylinder applications like V-8 and V-6 where there are multiple intake ports constantly drawing the mixture in. Most bikes and cars now have sequential/multipoint/whatever injection (except some Triumphs and Guzzis, NC700X.750X twins, and Gold Wings), meaning one injector per cylinder.

With that, there would be a waste of fuel half the time. It wouldn't be a 50% waste of the fuel, but still noticeable. Bikes with the cam pulse signal generator do not get markedly better mileage.

If there was a waste "squirt", you could swap the injector connectors for #2 & #3 cylinders, and it should run the same.