add hysteresis to limp rpm, boost, injector duty (#4541)

* add hysteresis

* changelog

firmware/CHANGELOG.md

@@ -31,6 +31,7 @@ Release template (copy/paste this for new release):
  - Manual electronic throttle synchronization #3680
  - Delay before enabling AC compressor #4502
  - Require full sync for odd cylinder count #4533
+ - Hysteresis on some fuel cuts to avoid engine damage #4541
 
 ### Fixed
  - Inverted vvt control #4464

firmware/controllers/limp_manager.cpp

@@ -45,7 +45,8 @@ void LimpManager::updateState(int rpm, efitick_t nowNt) {
 			? interpolate2d(Sensor::get(SensorType::Clt).value_or(0), engineConfiguration->cltRevLimitRpmBins, engineConfiguration->cltRevLimitRpm)
 			: (float)engineConfiguration->rpmHardLimit;
 
-		if (rpm > revLimit) {
+		// Require 50 rpm drop before resuming
+		if (m_revLimitHysteresis.test(rpm, revLimit, revLimit - 50)) {
 			if (engineConfiguration->cutFuelOnHardLimit) {
 				allowFuel.clear(ClearReason::HardLimit);
 			}
@@ -72,8 +73,10 @@ void LimpManager::updateState(int rpm, efitick_t nowNt) {
 	}
 
 	// Limit fuel only on boost pressure (limiting spark bends valves)
-	if (engineConfiguration->boostCutPressure != 0) {
-		if (Sensor::getOrZero(SensorType::Map) > engineConfiguration->boostCutPressure) {
+	float mapCut = engineConfiguration->boostCutPressure;
+	if (mapCut != 0) {
+		// require drop of 20kPa to resume fuel
+		if (m_boostCutHysteresis.test(Sensor::getOrZero(SensorType::Map), mapCut, mapCut - 20)) {
 			allowFuel.clear(ClearReason::BoostCut);
 		}
 	}
@@ -118,7 +121,8 @@ void LimpManager::updateState(int rpm, efitick_t nowNt) {
 
 	// If duty cycle is high, impose a fuel cut rev limiter.
 	// This is safer than attempting to limp along with injectors or a pump that are out of flow.
-	if (getInjectorDutyCycle(rpm) > 96.0f) {
+	// only reset once below 20% duty to force the driver to lift
+	if (m_injectorDutyCutHysteresis.test(getInjectorDutyCycle(rpm), 96, 20)) {
 		allowFuel.clear(ClearReason::InjectorDutyCycle);
 	}
 

firmware/controllers/limp_manager.h

@@ -54,6 +54,23 @@ struct LimpState {
 	}
 };
 
+class Hysteresis {
+public:
+	// returns true if value > rising, false if value < falling, previous if falling < value < rising.
+	bool test(float value, float rising, float falling) {
+		if (value > rising) {
+			m_state = true;
+		} else if (value < falling) {
+			m_state = false;
+		}
+
+		return m_state;
+	}
+
+private:
+	bool m_state = false;
+};
+
 class LimpManager {
 public:
 	// This is called from periodicFastCallback to update internal state
@@ -78,6 +95,10 @@ public:
 private:
 	void setFaultRevLimit(int limit);
 
+	Hysteresis m_revLimitHysteresis;
+	Hysteresis m_boostCutHysteresis;
+	Hysteresis m_injectorDutyCutHysteresis;
+
 	// Start with no fault rev limit
 	int32_t m_faultRevLimit = INT32_MAX;
 

unit_tests/tests/test_deadband.cpp

@@ -36,3 +36,20 @@ TEST(Deadband, InsideDeadband) {
 	// Now it should flip back
 	EXPECT_TRUE(d.gt(0, -5.01));
 }
+
+TEST(Hysteresis, basic) {
+	Hysteresis h;
+
+	// Below 'rising', should stay false
+	EXPECT_FALSE(h.test(15, 30, 20));
+	EXPECT_FALSE(h.test(25, 30, 20));
+
+	// over 'rising', should go true
+	EXPECT_TRUE(h.test(31, 30, 20));
+
+	// drop back below 'rising', should stay true
+	EXPECT_TRUE(h.test(25, 30, 20));
+
+	// drop below 'falling', should go false
+	EXPECT_FALSE(h.test(15, 30, 20));
+}

unit_tests/tests/test_limp.cpp

@@ -106,13 +106,18 @@ TEST(limp, boostCut) {
 	dut.updateState(1000, 0);
 	EXPECT_TRUE(dut.allowInjection());
 
-	// Above threshold, injection cut
-	Sensor::setMockValue(SensorType::Map, 120);
+	// Above rising threshold, injection cut
+	Sensor::setMockValue(SensorType::Map, 105);
 	dut.updateState(1000, 0);
 	EXPECT_FALSE(dut.allowInjection());
 
-	// Below threshold, should recover
-	Sensor::setMockValue(SensorType::Map, 80);
+	// Below rising threshold, but should have hysteresis, so not cut yet
+	Sensor::setMockValue(SensorType::Map, 95);
+	dut.updateState(1000, 0);
+	EXPECT_FALSE(dut.allowInjection());
+
+	// Below falling threshold, fuel restored
+	Sensor::setMockValue(SensorType::Map, 79);
 	dut.updateState(1000, 0);
 	EXPECT_TRUE(dut.allowInjection());