View Single Post
Old 03-25-2012, 03:48 PM   #67
roger 04 rt OP
Beastly Adventurer
roger 04 rt's Avatar
Joined: Nov 2011
Location: Massachusetts
Oddometer: 2,107
PS, Thanks for your interest and thoughtful feedback. In the quote section below, I've added some thoughts and comments. And the Wall-Adhered Fuel is good reading, and especially relevant when the engine is cold. However, even when warm there can be puddling, with an average evaporation rate of the puddle that has to be taken into account by the FI system. My comments below assume a tuned and air-balanced engine. RB

Originally Posted by Poolside View Post

Although the article we're discussing is using aviation as an example, I see it as completely relevant to our boxers.

... But the injector issue that causes a surging problem on the 1100/1150 isn't really related to the injectors. It relates to batch injection verses sequential injection.
... Agree that sequential would be better, but it seems to me that imbalance is more important.

In addition, the motor in an aircraft at cruise is operating at a very high 'percentage of full power' setting. Certainly a very high power setting compared to that of a road vehicle at highway speeds, which is closer to 15%.
...True, but whatever level the power a 10% fluctuation is still 10% of what's required by the vehicle in those conditions. And therefore felt.

Any injector imbalance on an aircraft motor is amplified by the the continuous flow rate, and the high power settings. Any imbalance error is multiplied 10 fold at least. It's a cumulative error, you know?
... Not convinced about that, I offer some thoughts on how fueling modulation by the Motronic could lead to asymetrical power fluctuations.

Injector imbalance on a road vehicle at highway speeds or lower doesn't make too much of a difference. The low power settings and intermittent injection period limits the accumulated error. Injector imbalance makes a little difference in smoothness, but it isn't a source of surging.
... On our two cylinder Boxers, if one cylinder is leaner than the other, three things can happen in closed loop: 1) Because the average has to be 14.7, one cylinder is leaner than the other meaning one has an excess of oxygen and the other is depleted. The spread between them is related to the fuel injector imbalance in a tuned engine. 2) The time for the Motronic program to go from lean to rich and rich to lean is extended because only one cylinder is contributing the oxygen that the Lambda sensor responds to. 3) As the time for the cycle goes up, the fuel ramp gets larger, meaning there is a greater modulation of the fuel supply.

The consequence in the leaner cylinder to the longer higher ramp of fuel, is a modulation of that cylinder's power output. While that is happening, the richer cylinder, depleted of oxygen, has an almost insignificant change in its power. The peak to peak variation of fuel can reach 8% or more (especially if the Lambda sensor is old and slow). So the leaner cylinder's power is fluctuating several percent. This can be felt.

But all of the above aside, and as you've experienced, if the fuel mixture is increased a little it cures a lot of ills.

roger 04 rt is online now   Reply With Quote