From 9e6a82e0bfe9cf0a884dc110293f497aa21bdb6d Mon Sep 17 00:00:00 2001
From: Sam Anthony <sam@samanthony.xyz>
Date: Sun, 2 Mar 2025 14:24:56 -0500
Subject: calculate mass flow rate at ambient conditions

---
 engine.c      | 16 ++++++++++++++++
 engine.h      |  1 +
 test.c        |  1 +
 test.h        |  1 +
 test_engine.c | 23 +++++++++++++++++++++--
 5 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/engine.c b/engine.c
index ad1932e..d0a1b38 100644
--- a/engine.c
+++ b/engine.c
@@ -17,6 +17,9 @@ static const double C_V_AIR = 718.0;
 /* Heat capacity ratio of dry air at T=300K [J/(kg*K)]. */
 static const double GAMMA_AIR = C_P_AIR / C_V_AIR;
 
+/* Gas constant of air [J/(kg*K)]. */
+static const double R_AIR = 287.05;
+
 
 static VolumeFlowRate port_volume_flow_rate(const Engine *e);
 static double density_ratio(const Engine *e);
@@ -100,3 +103,16 @@ density_ratio(const Engine *e) {
 	t3 = manifold_temperature(e);
 	return (p3 * t1) / (p1 * t3);
 }
+
+/* Mass flow rate through the engine (corrected to ambient conditions). */
+MassFlowRate
+mass_flow_rate(const Engine *e) {
+	Pressure p;
+	VolumeFlowRate v;
+	Temperature t;
+
+	p = e->ambient_pressure;
+	v = volume_flow_rate(e);
+	t = e->ambient_temperature;
+	return (p * v) / (R_AIR * t);
+}
diff --git a/engine.h b/engine.h
index f27220c..92b8b68 100644
--- a/engine.h
+++ b/engine.h
@@ -16,3 +16,4 @@ double pressure_ratio(const Engine *e);
 Temperature comp_outlet_temperature(const Engine *e);
 Temperature manifold_temperature(const Engine *e);
 VolumeFlowRate volume_flow_rate(const Engine *e);
+MassFlowRate mass_flow_rate(const Engine *e);
diff --git a/test.c b/test.c
index 592092b..460c320 100644
--- a/test.c
+++ b/test.c
@@ -62,4 +62,5 @@ main(void) {
 	test_comp_outlet_temperature();
 	test_manifold_temperature();
 	test_volume_flow_rate();
+	test_mass_flow_rate();
 }
diff --git a/test.h b/test.h
index fc1865c..9e127f6 100644
--- a/test.h
+++ b/test.h
@@ -68,3 +68,4 @@ void test_comp_outlet_temperature_adiabatic(void);
 void test_comp_outlet_temperature(void);
 void test_manifold_temperature(void);
 void test_volume_flow_rate(void);
+void test_mass_flow_rate(void);
diff --git a/test_engine.c b/test_engine.c
index 76e58fa..4acfa17 100644
--- a/test_engine.c
+++ b/test_engine.c
@@ -72,7 +72,7 @@ void
 test_volume_flow_rate(void) {
 	Pressure p_ambient, p_boost, map;
 
-	p_ambient = millibar(982.052713333343);
+	p_ambient = inch_mercury(30);
 	p_boost = psi(10);
 	map = p_ambient + p_boost;
 	Engine e = {
@@ -84,5 +84,24 @@ test_volume_flow_rate(void) {
 		.ve = percent(80),
 		.comp_efficiency = percent(65),
 	};
-	test(volume_flow_rate(&e), cubic_metre_per_sec(0.1855992));
+	test(volume_flow_rate(&e), cubic_metre_per_sec(0.184086));
+}
+
+void
+test_mass_flow_rate(void) {
+	Pressure p_ambient, p_boost, map;
+
+	p_ambient = inch_mercury(30);
+	p_boost = psi(10);
+	map = p_ambient + p_boost;
+	Engine e = {
+		.displacement = cubic_inch(250),
+		.rpm = rpm(5000),
+		.map = map,
+		.ambient_temperature = fahrenheit(70),
+		.ambient_pressure = p_ambient,
+		.ve = percent(80),
+		.comp_efficiency = percent(65),
+	};
+	test(mass_flow_rate(&e), kilo_per_sec(0.2214056));
 }
-- 
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