Engine Speed Control Observation
Your methodology there is somewhat flawed. Not only will you never see that Counter1 >= 20 (since you immediately set it to 0 before returning the count), but if you never get a pulse (engine is stalled) you'll never reach 20 pulses anyway.

A better method is to reverse everything and instead of timing how long it takes to get 20 pulses, count the number of pulses you get in a pre-defined time period.

Your interrupt remains the same, counting each pulse as it arrives, but you then sit and watch the clock (using millis()) and when a certain (to be decided by you) time has elapsed (the sample time) you record the pulse count and reset it to 0. You then have a number of rotations per time period.

If you set that time period to be 1 second (the simplest) then you immediately have the number of rotations per second.

void loop() {
    static uint32_t ts = millis();
    uint32_t pulses;

    if (millis() - ts &gt;= 1000) {
        ts += 1000;
        uint8_t sr = SREG;
        noInterrupts();
        pulses = Counter1;
        Counter1 = 0;
        SREG = sr;
        Serial.print(pulses);
        Serial.println(" revolutions per second");
    }
}


Decreasing the sample time means you can respond faster, but you are working with smaller numbers and hence less accuracy. For instance, setting it to 100ms instead of 1 second you need to multiply your count by 10 to get the current pulses per second. It all depends on how fast your engine runs as to what a good sample time would be.