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The retrofit of agricultural diesel engines to LPG–diesel dual-fuel operation is a practical pathway for reducing selected exhaust emissions, but simulation-based operating-window selection must be supported by engine-test bench evidence. This paper presents a complete experimental assessment of a naturally aspirated UTB 2404055 direct-injection diesel engine operated with diesel pilot injection and intake-supplied LPG. The test program covered a full operating matrix defined for the retrofit study, including three LPG energy fractions, 0%, 20%, and 40%, two engine speeds, 1400 and 2400 rpm, and four load levels, 40%, 60%, 80%, and 100%. For each operating point, effective performance and exhaust-emission responses were measured and compared with the corresponding diesel-only baseline as a reference.
The experimental results show that the main emission tendencies expected for LPG-assisted diesel operation are generally preserved under real engine conditions. Smoke/soot emissions are reduced in most LPG–diesel cases, while the carbon monoxide response remains strongly dependent on speed and load. The NOx response is not uniform across the operating map: reductions are observed only in selected speed–load–LPG combinations, whereas other regions show neutral or unfavorable behavior relative to diesel-only operation. The experimental map also indicates similar BSFC to reference under LPG–diesel operation, showing that the retrofit assessment must be treated as an operating-window problem rather than as a fuel-substitution effect alone.