engine-design

name	engine-design
description	Engine design — thermodynamic cycles (Otto, Diesel, Atkinson), BMEP, BSFC, volumetric efficiency, bore-stroke ratio, valve timing, combustion analysis, turbocharging, friction estimation, SAE J1349/ISO 1585.
metadata	{"priority":7,"promptSignals":{"phrases":["engine design","Otto cycle","Diesel cycle","BMEP","volumetric efficiency","engine combustion"],"minScore":3}}

Engine Design — Complete Skill

Thermodynamic Cycles

Otto Cycle (Spark Ignition, Gasoline)

Thermal efficiency: η_th = 1 - 1/r_c^(γ-1) [r_c = compression ratio; γ = ratio of specific heats = 1.35–1.40 for air-fuel] At r_c = 10: η_th = 1 - 1/10^0.4 = 1 - 0.398 = 0.602 = 60.2% (ideal Otto; actual ~35%)

Practical limit: knock limits r_c to 10–13:1 (gasoline); higher with direct injection, octane boost

Diesel Cycle

Thermal efficiency: η_th = 1 - (r_c^γ / r_c^γ) × (r_e^γ - 1) / (γ(r_e - 1)) [r_e = cutoff ratio = V3/V2] Or: η_th = 1 - (1/r_c^(γ-1)) × (r_e^γ - 1) / (γ × (r_e - 1)) At r_c = 17, r_e = 2: η_th ≈ 63% ideal; actual ~45%

Diesel advantage: higher r_c (no knock limitation) → higher η_th; lean operation → no throttle losses

Atkinson / Miller Cycle

Extended expansion: late IVC (intake valve closing) → effective compression ratio < expansion ratio → higher thermal efficiency; less work out per cycle but better efficiency Application: Toyota Prius hybrid; gasoline engines with variable valve timing

Key Performance Metrics

Brake Mean Effective Pressure (BMEP)

BMEP = Net work per cycle / displacement volume: BMEP = T_brake × (2π × 2/n_cylinders) / V_d [4-stroke: divide by 2 for 2 revolutions] = P_brake / (V_d × N × n_strokes/2) × 60 [kPa; P = power in kW; N = RPM; V_d in liters]

Typical BMEP values:

Engine type	BMEP [kPa]
Naturally aspirated gasoline	900–1200
Turbocharged gasoline	1400–2500
Naturally aspirated diesel	700–1000
Turbocharged diesel	1400–2000
Racing (F1, high boost)	3000–4000

Specific power (power density): P_specific = BMEP × V_d × N / (2 × 60,000) [kW/L] High-output: > 100 kW/L (turbocharged gasoline); diesel: 50–80 kW/L

Brake Specific Fuel Consumption (BSFC)

BSFC = fuel_mass_flow / power: BSFC = ṁ_fuel / P_brake [g/kWh]

Typical values:

Engine type	BSFC [g/kWh]
Naturally aspirated gasoline	250–280
Modern DI gasoline	230–260
Turbocharged diesel	200–230
Diesel heavy-duty (best)	175–195
Gas turbine	300–400

Relation to thermal efficiency: η_brake = 3600 / (BSFC × LHV_fuel/1000) [LHV_gasoline ≈ 43,600 kJ/kg; LHV_diesel ≈ 42,700 kJ/kg] BSFC = 200 g/kWh diesel: η = 3600/(200×42.7/1000) = 3600/8540 = 42.2%

Volumetric Efficiency

η_v = actual air mass / ideal air mass: η_v = ṁ_air / (ρ_ambient × V_d × N/2) [for 4-stroke; N/2 = cycles per second] Typical: 0.80–0.90 (naturally aspirated); > 1.0 (supercharged/turbocharged)

Factors affecting η_v:

• Valve timing (cam profile; VVT improves ηv across speed range)
• Port flow coefficient (C_d of intake valve; 0.3–0.7 depending on lift)
• Charge heating (hot intake runner → density drop)
• Back-pressure (exhaust restriction)

Bore-Stroke Ratio

Square engine: bore = stroke → balanced between breathing and friction Over-square (short stroke): bore > stroke → better valve area / displacement; lower piston speed → high revving; less friction Under-square (long stroke): stroke > bore → higher BMEP (longer moment arm); better low-speed torque; limited by piston speed

Mean piston speed: v_p_mean = 2 × stroke × N / 60 [m/s; N in RPM] Limit: v_p ≤ 12–15 m/s (production); 25 m/s (racing)

Bore-stroke ratio (B/S): Production gasoline: B/S = 1.0–1.2 (slightly over-square) Racing: B/S = 1.3–1.5 (over-square; high rev limit) Diesel: B/S = 0.9–1.1 (near-square or slightly under-square)

Valve Timing

4-stroke timing events: IVO = intake valve open; IVC = intake valve close; EVO = exhaust valve open; EVC = exhaust valve close Valve overlap: IVO-EVC region; enhances scavenging at high RPM; increases HC at idle

Optimal IVC for maximum power: IVC_optimal = ABDC 50–80° (after BDC): Intake charge inertia continues after BDC → ramscharging VVT (Variable Valve Timing): advance/retard cam → optimize IVC for each RPM → wider power band

Valve lift and flow: C_d = flow coefficient (bench-tested); A_throat = π/4 × D_v² × sin(30°) × L_v / D_v (curtain area approximation) Mass flow: ṁ = C_d × A × (ΔP_throat) × √(2ρ/ΔP) [limited by choked flow at high lift]

Combustion Analysis

Indicated mean effective pressure (IMEP): IMEP = ∫ P dV / V_d [kPa; from measured P-V diagram] FMEP = friction mean effective pressure ≈ IMEP - BMEP PMEP = pumping mean effective pressure ≈ -∫P dV (exhaust-intake)

Friction estimation: FMEP ≈ a + b × v_p + c × BMEP [empirical Heywood correlation; a, b, c from data] FMEP_motored ≈ 60 + 0.6 × v_p + 0.005 × BMEP [kPa; rough estimate]

Combustion phasing: CA50 (crank angle of 50% heat release): optimal for η at CA50 = 5–10° ATDC MBT (minimum spark advance for best torque): determined by CA50 target

Knock: auto-ignition of end gas before flame arrival; detected by vibration sensor; retard timing to prevent

Turbocharging

Pressure ratio: PR = P_boost / P_ambient [gauge boost pressure [kPa] + 101.3 / 101.3] E.g., 1 bar boost → PR = 2.0

Compressor map: Corrected flow: Q_corr = Q_actual × √(T_in/T_ref) × (P_ref/P_in) Corrected speed: N_corr = N_actual / √(T_in/T_ref) Operating point must be within compressor map; left of surge line; below choke line

Turbine matching: Turbine inlet T estimation: T_turbine_in = T_exhaust_manifold ≈ T_EGT = f(AFR, load, timing) Expansion ratio across turbine: determined by exhaust back-pressure and PR_comp (waste-gated for control)

Boost response (turbo lag): Lag ≈ rotor inertia / turbine power → reduce inertia (variable turbine geometry, twin-scroll) → faster spool

Standards

Standard	Scope
SAE J1349	Engine power test code (gasoline, diesel)
ISO 1585	Road vehicles — engine test codes
SAE J1826	Turbocharger nomenclature
DIN 70020	Engine power measurement (German)
ISO 3046	Reciprocating internal combustion engines

Output

Provide: engine type (SI/CI), cycle (Otto/Diesel/Atkinson), bore × stroke [mm], displacement V_d [L], compression ratio r_c, ideal η_th [%] and actual brake η [%], BMEP [kPa] at rated power, rated power and torque [kW, N·m at RPM], BSFC at best point [g/kWh], volumetric efficiency η_v [%], mean piston speed [m/s] vs. limit, CA50 phasing [°ATDC], FMEP [kPa] at rated speed, turbocharged (PR, boost [kPa], T_compressor_out [°C]), emission system requirements, and applicable standard (SAE J1349, ISO 3046).