How to Diagnose your Vehicle if the Fuel Consumtion Increase? 1of 2


Increased fuel consumption frequently is a result of the vehicle usage and road conditions rather than vehicle malfunction. Therefore, it is important to accurately understand the customer’s habitual usage and required level. In this chapter, the items regarding the estimated causes of increased fuel consumption are explained.
• Since when?
Understand the relationship between engine warm-up and A/C usage, the change of the vehicle condition and the malfunction.
• Compared to what?
Find the difference between the target vehicle that the customer compares and the customer’s vehicle and troubleshoot the cause of the increased fuel consumption.
• How is it used?
Troubleshoot the cause of the increased fuel consumption based on the customer’s usage.
• How is it measured?
Troubleshoot the cause of the customer’s measurement error.
Since When?
1. Seasonal variation of fuel consumption
When the air conditioner is used in summer, fuel consumption increases. The degree of increase depends on the air conditioner load which is affected by the temperature and moisture. In winter, the fuel consumption also increases, because fast idling is performed for longer than the usual in order to warm up the engine.
2. Fuel consumption change over time
Knocking occurs because carbon in the combustion chamber accumulates for a long period of time. Knock control retards the ignition timing increasing fuel consumption. If the ignition timing retards by fi ve degrees, the fuel consumption increases by approximately 6%. When a brand new vehicle runs approximately 5,000 to 10,000 km, the fuel consumption decreases by 5 to 10 %. This is because the friction to the engine, drive train, tires, etc. is reduced.
3. Rapid change in the fuel consumption
“Fuel consumption has increased a lot compared to last year,” “Suddenly, the fuel consumption has increased, ” a sort of malfunction may have occurred to the vehicle.
Compared to What?
1. Difference in the engine
• In general, fuel consumption is greater when the engine displacement is larger. The main cause being that friction increases as the engine is larger and the vehicle weight is heavier.
• The fuel consumption in the normally used range is almost the same despite of the characteristics of the engine; an engine that has great torque at a low speed and that has great at a high speed. The gear ratio of the engine that has great torque at a low speed is set smaller, however, in this case, the fuel consumption can be said to be decreased.
• In an engine equipped with a turbocharger/supercharger, the driver frequently
depresses the accelerator pedal more than is necessary, because of its slow reaction when moment of starting and accelerating. As a result, the turbocharger/ supercharger functions, starting the acceleration to be greater than is necessary and the fuel consumption increases.
Minimum fuel consumption rate
The minimum fuel consumption rate shown by the engine performance curve is when the throttle valve is fully opened and it is not always equal to the fuel consumption in the normally used range.
2. Vehicle weight difference
As the vehicle weight is greater, fuel consumption increases. The increase in vehicle weight does not greatly affect the fuel consumption while driving at a constant speed on a fl at road. But, when repetitively starting, accelerating and climbing up hill, the vehicle weight largely affects the fuel consumption.
3. Difference in style (aerodynamics)
The air resistance increases against the square of the vehicle speed. Therefore, in low-speed driving, the air resistance does not affect the fuel consumption, but in high-speed driving, it largely affects the fuel consumption.
The air resistance is in proportion to the multiplication of the Cd value of the frontal projected area. In other words, even if the Cd value is small, the frontal projected area is large, so the air resistance becomes large. Therefore, even if the Cd value in a large vehicle is small, the air resistance is not always small. In short, it cannot be said that the fuel consumption will decrease.
Compared to What?
4. Difference in transmission and gear ratio
• Generally, when the gear ratio is small, engine speed is kept low and the throttle valve opening becomes large to drive with the equal power. As a result, the engine pumping loss decreases and fuel consumption decreases.
• When comparing a manual transmission vehicle (hereafter referred to as an M/T vehicle) with an automatic transmission vehicle (hereafter referred to as an
A/T vehicle), in low-speed driving, the fuel consumption of an A/T vehicle is larger than that of an M/T vehicle due to torque converter slip. On the other hand, in high speed driving where the lock-up functions, the fuel consumption in both vehicles becomes equal.
Pumping loss
Pumping loss is the resistance when the engine sucks in the air. When the throttle valve opening is small, pumping loss is large.
5. Tire difference
The reason why the tire affects the fuel consumption is that the majority of the rolling resistance during running resistance is in the tire. The tire rolling resistance varies depending on the air pressure or tire type.
6. Difference between the actual vehicle and the catalogue data
The fuel consumption in the catalogue is measured under a specified condition. Therefore, in many cases, the running conditions are different from those performed by customers, the fuel consumption increases depending on the conditions; average running speed is low, the ratio of vehicle stoppage is high, sudden acceleration is performed, etc.
• Causes due to roads and surroundings
Temperature and moisture are different Wind influences Speed changes considerably due to slope, inclination and humps on the roads
• Cause on the vehicle side
Vehicle weight is different
Keeping the throttle opening constant is difficult even during constant running Change with over time (Vehicle friction change, carbon accumulation)

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