Öz

In this study, a simulation model has been developed for the Otto-cycle to be used in the spark ignition engines using the finite-time thermodynamics method. In the simulation model, it has been assumed that the temperature dependent-specific heats according to a logarithmic function, that the initial cycle temperature is higher than the ambient temperature with the effect of the residual gas temperature, irreversibilities in the compression and expansion processes, and heat, combustion and friction losses and the working fluid is composed of fuel-air-residual gases. The effects of compression ratio, equivalence ratio, and stroke/bore ratio on engine performance were investigated in detail. Thermal efficiency, specific fuel consumption, specific fuel cost and power density were used for performance analysis. Also, it was accepted that volumetric efficiency decreased when LPG was used in the engine. Performance loss factors were used for engine heat balance. The characteristics of a single-cylinder spark ignition engine have been used as a reference in the simulation. As a result of the comprehensive numerical study, it was observed that the power density decreased by 12% when the volume utilization of LPG decreased by 10%. Although the specific fuel consumption of LPG is higher than that of gasoline, the specific fuel cost of LPG is very low compared to gasoline due to the LPG/gasoline price ratio. The loss of performance decreases as the compression ratio increases and the stroke/bore ratio decreases. Also, performance losses increase when the equivalence ratio is greater than 1. It is seen that LPG performance can be eliminated by increasing the volumetric efficiency or increasing the compression ratio. When the LPG/gasoline price ratio is 0.54, it is determined that the engine running on LPG is 24% more economical compared to working with gasoline. When the LPG/gasoline price ratio is 0.67, it has been observed that LPG has no economic advantage. With this study, significant results have been obtained especially for engine designers.

Anahtar Kelimeler

• LPG-gasoline comparision
• performance analysis
• spark-ignition engine
• engine simulation
• Otto-cycle

Yakıt olarak benzin ve LPG kullanılan buji ateşlemeli bir motorun simülasyonu ve performans analizi

Öz

Bu çalışmada sonlu zaman termodinamiği metodu kullanılarak buji ateşlemeli motorlarda kullanılmak üzere, Otto çevrimi için bir simülasyon modeli oluşturulmuştur. Simülasyon modelinde, logaritmik bir fonksiyona göre özgül ısıların sıcaklığa bağlı değiştiği, çevrim başlangıç sıcaklığının artık gaz sıcaklığının etkisi ile ortam sıcaklığından yüksek olduğu, sıkıştırma genişleme süreçlerinde tersinmezliklerin olduğu ve ısı, yanma ve sürtünme kayıplarının olduğu, çalışma maddesinin yakıt-hava-artık gaz karışımından oluştuğu kabul edilmiştir. Sıkıştırma oranı, eşdeğerlik oranı ve strok/çap oranının motor performansına etkisi detaylı olarak incelenmiştir. Performans analizi için ısıl verim, özgül yakıt tüketimi, özgül yakıt maliyeti ve güç yoğunluğu kullanılmıştır. Ayrıca motorda LPG kullanıldığında hacimsel verimin azaldığı kabul edilmiştir. Motor ısı balansı için performans kaybı faktörleri kullanılmıştır. Tek silindirli buji ateşlemeli bir motorun özellikleri referans olarak simülasyonda kullanılmıştır. Yapılan kapsamlı sayısal çalışma neticesinde, LPG kullanımında hacimsel verimin %10 azaldığı kabul edildiğinde güç yoğunluğunun %12 azaldığı görülmüştür. LPG’nin özgül yakıt tüketimi benzine göre yüksek olmasına rağmen LPG/benzin fiyat oranı nedeniyle LPG’nin özgül yakıt maliyeti benzine göre oldukça düşük olmaktadır. Sıkıştırma oranının artmasıyla ve strok/çap oranının azalmasıyla birlikte performans kayıpları azalmaktadır. Ayrıca eşdeğerlik oranı 1’den büyük olduğunda performans kayıpları artmaktadır. LPG’nin performans düşüklüğü özellikle hacimsel verimin artırılması veya sıkıştırma oranının artırılmasıyla bertaraf edilebileceği görülmektedir. LPG/benzin fiyat oranı 0,54 olduğunda LPG ile çalışan motorun benzin ile çalışmasına kıyasla %24 daha ekonomik olduğu belirlenmiştir. LPG/benzin fiyat oranı 0,67 olduğunda ise LPG’nin ekonomik avantajının olmadığı görülmüştür. Bu çalışmayla birlikte özellikle motor tasarımcıları için önemli sonuçlar elde edilmiştir.

Anahtar Kelimeler

• LPG-benzin karşılaştırması
• performans analizi
• buji ateşlemeli motor
• motor simülasyonu
• otto çevrimi

Kaynakça

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