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#include <math.h>
#include <string.h>
#include "unit.h"
#include "engine.h"
/* A four-stroke piston engine takes two revolutions per cycle. */
static const double REV_PER_CYCLE = 2.0;
/* Specific heat of dry air at constant pressure at T=300K [J/(kg*K)]. */
static const double C_P_AIR = 1005.0;
/* Specific heat of dry air at constant volume at T=300K [J/(kg*K)]. */
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);
void
init_engine(Engine *e) {
memset(e, 0, sizeof(*e));
}
Pressure
comp_outlet_pressure(const Engine *e) {
return e->map + e->intercooler_deltap;
}
/* Pressure ratio across the compressor. */
double
pressure_ratio(const Engine *e) {
Pressure p1, p2;
p1 = e->ambient_pressure;
p2 = comp_outlet_pressure(e);
return p2 / p1;
}
Temperature
comp_outlet_temperature(const Engine *e) {
Temperature t1, dt;
Pressure p1, p2;
double exp;
t1 = e->ambient_temperature;
p1 = e->ambient_pressure;
p2 = comp_outlet_pressure(e);
exp = (GAMMA_AIR - 1.0) / GAMMA_AIR;
dt = t1 * (pow(p2/p1, exp) - 1.0) / e->comp_efficiency;
return t1 + dt;
}
Temperature
manifold_temperature(const Engine *e) {
Temperature t1, t2;
t1 = e->ambient_temperature;
t2 = comp_outlet_temperature(e);
return t2 - (t2 - t1)*e->intercooler_efficiency;
}
/* Volume flow rate throught the compressor inlet. */
VolumeFlowRate
volume_flow_rate(const Engine *e) {
VolumeFlowRate v3;
double r;
v3 = port_volume_flow_rate(e);
r = density_ratio(e);
return v3 * r;
}
/* Volume flow rate through the intake ports. */
static VolumeFlowRate
port_volume_flow_rate(const Engine *e) {
double n, d, ve;
n = as_rpm(e->rpm);
d = as_cubic_metre(e->displacement);
ve = e->ve;
return cubic_metre_per_min(n * d * ve / REV_PER_CYCLE);
}
/* Density ratio between the ports and the compressor inlet. */
static double
density_ratio(const Engine *e) {
Pressure p1, p3;
Temperature t1, t3;
p1 = e->ambient_pressure;
p3 = e->map;
t1 = e->ambient_temperature;
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);
}
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