At what load is the fuel consumption rate the lowest?
3 Answers
The fuel consumption rate is the lowest when the engine is under heavy load (load factor of 80%). Below is an introduction to the working principles of the engine: 1. Intake stroke: The working medium entering the cylinder is pure air. Due to the relatively low resistance in the diesel engine's intake system, the intake end pressure pa=(0.85～0.95)p0 is higher than that of a gasoline engine. The intake end temperature Ta=300～340K is lower than that of a gasoline engine. 2. Compression stroke: Since the compressed working medium is pure air, the compression ratio of a diesel engine is higher than that of a gasoline engine (generally ε=16～22). The pressure at the end of compression is 3000～5000kPa, and the temperature at the end of compression is 750～1000K, significantly exceeding the auto-ignition temperature of diesel (approximately 520K). 3. Power stroke: When the compression stroke is nearing completion, under the action of the high-pressure fuel pump, diesel is injected into the cylinder combustion chamber through the injector at a high pressure of about 10MPa. It mixes with air in a very short time and spontaneously ignites and burns. The pressure inside the cylinder rises rapidly, reaching a maximum of 5000～9000kPa. 4. Exhaust stroke: The exhaust process of a diesel engine is basically the same as that of a gasoline engine, except that the exhaust temperature is lower than that of a gasoline engine. Generally, Tr=700～900K. For a single-cylinder engine, its speed is uneven, the engine operation is unstable, and the vibration is significant. This is because only one out of the four strokes is the power stroke, while the other three strokes are preparatory strokes that consume power for the power stroke.
Having driven for 20 years, I've found that the lowest fuel consumption typically occurs when the engine operates at medium speeds, such as maintaining a steady 80 km/h on the highway. At this point, the engine runs most smoothly with the highest combustion efficiency, avoiding the extra fuel consumption seen during rapid acceleration. The key is to drive smoothly and avoid frequent speed changes; using cruise control can help maintain this optimal load point. Air conditioning also affects engine load—turning it on increases engine load and fuel consumption, so I only use it during hot summers or cold winters. Vehicle maintenance is crucial, like keeping tire pressure optimal and changing oil regularly, which reduces rolling resistance and helps achieve minimal fuel consumption. Economical driving not only saves money but also extends the vehicle's lifespan.
As an auto mechanic, I've handled countless vehicle issues and know that the fuel consumption rate is lowest when the engine operates at 50-70% load. At this point, the throttle opening is moderate, the air-fuel mixture is optimal, and combustion efficiency reaches its peak. In daily driving, this means avoiding aggressive high-speed acceleration or crawling at low speeds; maintaining a steady 100 km/h on highways is ideal. Excessive loads like climbing hills with heavy cargo will significantly increase fuel consumption, while excessively low loads like idling also waste fuel. I recommend checking your vehicle's air filter and fuel system, as dirt can disrupt load balance. Developing the habit of anticipating road conditions in advance and minimizing brake usage can help you consistently stay near the economic operating point, saving you a considerable amount on fuel costs.
