I guess I will go ahead and give some facts on this saw to help dispel any myths or confusion about how it works. First of all I work for STIHL and I am not here to try to sell anybody anything but I do like working for STIHL and I like what we build and sell and I am not ashamed to say so.
The MS 500i is based on what was learned from the TS 500i. If you can make fuel injection work on a concrete saw reliably then you have accomplished something. The TS runs around 10k RPM, but this saw is up around 13k so a bit more of a challenge.
Here is how it works: pump the primer to build up pressure in the system. Once the engine is running fuel pressure is maintained with an impulse driven fuel pump with a dampner and a regulator. Pull the rope and the generator under the flywheel wakes up the computer (Electronic Control Unit). The ECU looks at the temp probe in the crankcase and compares the temp with the second probe in the kill switch. Now it knows if this is a cold start or warm start and adjusts fuel flow accordingly. There is no choke. Then it looks at the pressure sensor in the crankcase and sees the pressure go high when the piston is at BDC and low at TDC. This creates an up down signal that is then compared to the AC sign wave from the generator that is used as a tach and crank position sensor. So if the computer is happy with the input data then it cycles the injection valve. A 40hz square wave is used to open and close the injection solenoid at up to 30 times a second. The ignition system has variable timing. The fuel is released under the air-stream coming from the throttle body into the crankcase when the piston is almost at TDC, so operating pressure is around 1 to 1.4 PSI. The fuel is mixing with air in a negative pressure zone with a spinning crankshaft so atomization is complete before the mix enters the transfer ports. The cylinder is a delayed scavenge design so the port timing is such that a little of the blow down pressure from the combustion event pushes a bit of exhaust gas into the top of the 4 long, closed transfer ports to delay the incoming fuel charge just a fraction of a second to give the piston time to come up and close the ex port before any scavenge loss of fuel becomes excessive. For the EPA in USA 72 grams/kilowatt hour is the limit for this size engine and this saw comes in below that. This system automatically compensates for altitude, load, a dirty air filter and even variance in the quality of the fuel.
Some interesting facts that I share when I teach the Gold schools that I have found most techs are not aware of: at 9500 RPM the spark plug fires 158 times in one second. So in one second we have gone through the entire two-stroke process 158 times. After 1 hour of full throttle operation at around 9000 RPM the crankshaft will make over 500,000 revolutions. At full throttle this saw will flow over 12 cubic feet of air per minute.
This saws weighs about what a MS 362 does and has slightly more power than a MS 661, and as the man in the video said, it has really great throttle response. It truly is a wonder to cut with.
I hope this explanation helps those that are interested in understanding this new technology a bit better.
40hz is 40 cycles per second... but you say the injection solenoid operates at up to 30 times a second. So some of the square waves don’t trigger the injector opening?...this saw is up around 13k so a bit more of a challenge...A 40hz square wave is used to open and close the injection solenoid at up to 30 times a second...at 9500 RPM the spark plug fires 158 times in one second.