|02-20-2013, 04:01 AM||#11|
Joined: Aug 2007
Location: San Antonio, Texas
Oops, should have checked the link in Power-Tripp's post (it's no longer working). Here's another post to another forum (which also has the same broken link), but he explains the problem in a little more detail.
Finally found the column at another location:
EFI 102: The injector industry’s dirty little secrets
Throughout 2011, this column has covered some of the technical details of engine operation and support systems such as fuel, oil, cooling, the basics of engine management, etc. Now for 2012, it is time to dive into the things that we need to know to make the engine operate efficiently and perform best. With nearly all motorcycles coming off the showroom floor with fuel injection and electronic engine management, the days of carburetors are quickly com- ing to a close. Sad, but true. As a result, we need to become knowledgeable on the detailed parts that make electronic engine management work, and the first and foremost is the injector. Even though this is a subject on wet flow, the topic tends to be a fairly dry discussion. We will try to cut it down to its most basic parts.
A fuel injector is an electric solenoid. This means that it is a valve that uses an electromagnetic coil to open and allow fuel to fuel flow through it. In the simplest terms, an injector is nothing more than a tube that flows fuel from one end to the other. Inside the tube is a spring loaded shaft with a tapered end that fits tightly into a seat cut into the outlet end of the injector. An electric coil on the outside of the tube lifts the shaft off the seat and opens the injector when electricity flows into the coil. The spring pushes the shaft down against the seat as soon as electricity is turned off. This stops fuel flow. The amount of time that the injector coil is turned on is termed the injector pulse unit (IJPU) and can be measured in the percentage of on-time versus off-time – called duty-cycle. A 10 percent duty-cycle means that the injector is only turned on 10% of the time. And a 100% duty-cycle means that the injector is open all of the time.
Inside the ECU module are coil drivers that send the electric current to the injectors. These drivers and injector coils heat up when they flow current through them, as a result, the ECU software tends to shut them down when they exceed 85-87% duty-cycle to prevent heat damage to the drivers and injectors as well as the rest of the ECU. This sets the limit of useable flow for a given injector at a specific fuel pressure. As the injectors wear, and are exposed to operational heat from the engine and the injector coil itself, the flow changes at a specific duty-cycle.
If you take a set of brand new injectors out of the box and flow test them at 100% duty cycle, even production new injectors tend to all flow within 1-1.5% of each other. [WOW! A matched set right out of the box! Lucky me... yeah, RIGHT! Guess what... they all do that. No need to send them out for matching, the manufacturer has already done it for you at 100% duty-cycle. But this doesn’t mean much.]
At 100% is where most OEM and aftermarket injector companies tend to test and match injectors into sets. But as we have seen, the ECU cannot operate at 100% duty cycle without damage to the drivers or injectors over time. So why anyone would care about flow at 100% duty cycle is anyone’s guess. The fact that most injectors are flow tested with “test fluid” instead of actual fuel, and most use mineral spirits as their “test fluid” only makes things worse. Mineral spirits have a different density and viscosity than pump gas that results a 13% difference in actual flow. Race fuels vary even more, and the spray pattern with mineral spirits is different from that with fuels. Only a few select companies will actually test fuel injectors throughout their duty- cycle range (dynamic range testing) and match them up in sets for flow and spray pattern with specific fuel pressures, voltages, and fuel types.
[Some high flow injectors purchased from aftermarket suppliers are nothing more than stock injectors that have been taken apart and given a whack on the pintle shaft with a pin punch and large mallet to make them open further and flow more. Sad, but true.]
Since the injector is a mass produced part, it has certain production tolerances for the valve, valve seat, and the coil itself. So even though the injector is rated for a given amount of flow at a set pressure, this varies a bit more than many realize at less than 100% duty-cycle. Most OEM injectors have 25% to 40% overhead at 85% duty-cycle. This means they flow considerably more fuel than the engine can use in stock trim. This causes the engine to actu- ally operate at low duty-cycles, even at wide open throttle, at peak rpm. At low rpm and loads where we cruise, the duty-cycle percentage is very small. As a result of variations in electronic slew rate and electric lag time of the injector coil and driver (the amount of time it takes the injector to open fully), and the variances in machining tolerances of the needle and seat inside the injector, we now have more and more variation in how much a specific injec- tor flows at smaller duty cycles. Injector flow variations of 7%-14% are common, and as much as 30%-40% are not uncommon at cruise duty-cycles. [If the electrical system voltage changes due to charging, regulator, wiring, or battery issues, the injector flow changes. Less voltage means it takes longer for the injector to open, and the lag time takes up more of the injector on-time. The fuel pump produces varying pressures that alter injector flow, as well. Something to think about.]
What does this all mean to the average motorcycle rider? Well, it means that the manufacturer pays an individual to spend a lot of time and effort developing a factory calibration for each specific model using injectors that are flow matched throughout their duty-cycle range, to meet output, opera- tion, noise, and gas emission goals (among others). THEN, the stock calibration is offset for variations in fuels and injectors, as well as other variations throughout the world. This means that if we take 10 different bikes of the same make model and year, and test them at different engine speeds and loads, we get a wide range of results. This is on completely stock engines. If it were not for the stock narrow-band Lambda sensor (exhaust gas oxygen sensor), that the ECU uses to sense fueling at idle and cruise conditions, and to make offsets to the factory mapping or calibrations, most stock bikes would have rideability issues far from acceptable. In fact, many stock bikes are not even truly acceptable due to having to meet strict emission, mileage, and noise standards.
As we change the air flow into and out of the engine with aftermarket air filters, intake kits, headers, silencers, and more, the engine wants specific changes to fueling and ignition timing for best output, fuel consumption, and changes to throttle input (transient response). The good thing is that we can make these changes and give the engine what it wants, because we are not limited by the OEM constraints. If we know how to listen to what the engine is telling us, it will tell us what it wants and needs... all we have to do is listen carefully. Give the engine what it wants, not what you think it wants. The result in performance, transient response, fuel consumption, smooth operation, engine life, and more, are well worth taking the time to listen.
Isn’t it amazing how important injectors are to the operation of your engine?
Time to go, there is fuel to burn, tires to wear, and roads that are calling my name in the wind. I might even get a chance to try these new inexpensive grip heaters that Santa left in my stocking.
By Wayne Tripp
Power-Tripp Performance, Inc. www.Power-Tripp.com
Proud to have supported Neduro in the 2012 Dakar and Pyndon in the 2013 Dakar
2014 KTM 1190 Adventure R
2007 KTM 990 Adventure (Sold)
2008 KTM 300 XC (plated)
2008 KTM 690 Enduro
DirtJack screwed with this post 02-20-2013 at 04:23 AM Reason: Include Power-Tripp's column
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