Thermodynamics of the Internal Combustion Engine AGUS HARYANTO

PERSAMAAN 2 TERMODINAMIKA • Hukum Boyle untuk gas ideal: P 1 V 1 = P 2 V 2 = Pn. Vn = konstan • Hukum Charles: Pada P konstan, V 1/V 2 = T 1/T 2 Pada V konstan, P 1/P 2 = T 1/T 2 • Persamaan gas ideal P 1 V 1/T 1 = P 2 V 2/T 2 = … PV/T = R PV = RT PV = m. RT

PERNYATAAN Kelvin-Plank (1850) For heat engines: It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work.

BOTTOM LINE • Tidak ada mesin siklik yang memiliki efisiensi 100%

Panas spesifik, entropi, perubahan energi dakhil • Panas spesifik Q = m. Cp(T 2 – T 1) • Entropi T d. S = d. Q • Perubahan energi: Q = U 2 – U 1 + W

Diagram P-v dan T-s

Efisiensi Termal

cold-air-standard assumptions Air-standard assumptions: 1. The working fluid is air, which circulates in a closed loop and behaves as an ideal gas. 2. All the processes are internally reversible. 3. The combustion process is replaced by a heataddition from an external source 4. The exhaust process is replaced by a heatrejection and restores the fluid to initial state. • Air has constant specific heats at room temperature 25°C. cold-air-standard assumptions.

Siklus Motor Otto (bensin) Ideal The ideal Otto cycle consists of four internally reversible processes: • 1 -2 Isentropic compression • 2 -3 Constant-volume heat addition (Combustion) • 3 -4 Isentropic expansion (Langkah usaha) • 4 -1 Constant-volume heat rejection (langkah buang)

ENERGY BALANCE SIKLUS OTTO • (qin – qout) + (win – wout) = u • Qin = u 3 – u 2 = Cv(T 3 – T 2) • Qout = u 4 – u 1 = Cv(T 4 – T 1)

ENERGY BALANCE SIKLUS OTTO • Mean effective pressure (MEP): if it acted on the piston during the entire power stroke, would produce the same amount of net work as that produced during the actual cycle • Wnet = MEP * Vol. displcm

ENERGY BALANCE SIKLUS OTTO • r = compression ratio • k = heat specific ratio = Cp/Cv

Note: Compression ratio yang tinggi akan berakibat pada suhu kompresi yang tinggi juga. Pada motor bensin, jika suhu ini mengakibatkan AUTOIGNITION dan menghasilkan noise yang disebut KNOCKING. Efisiensi termal motor bensin aktual 25 - 30 %.