Idling Emissions

Hannu Jääskeläinen

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Abstract: Some categories of heavy-duty diesel engines, such as long-haul trucks or locomotives, are routinely idled for prolonged periods of time, resulting in idle emissions and fuel consumption. Idle emissions depend on a number of operational conditions, including the use of cabin heating and air conditioning, idle engine speed, and ambient temperature. They also depend on the vehicle technology.


The term idling refers to the continuous operation of a vehicle’s main propulsion engine while the vehicle is stopped. Idling is common in traffic conditions, especially during urban driving, such as at traffic lights or in stop-and-go driving during traffic congestion. However, idling periods in traffic are relatively short. There is more concern over long periods of idling of heavy-duty diesel engines while the vehicle is parked and not in active service. This may have more of an adverse environmental impact and be a source of significant additional—and often unnecessary—fuel consumption.

A suspected major source of idle emissions are long-haul trucks, which are routinely idled overnight, mainly to provide cab heating and air conditioning. In addition to heat and air conditioning, truck accessories such as stereos, CB radios, interior lights, televisions, computers and refrigerators demand power and can motivate idling even if climate control is not needed. In extreme cases, up to 6 kW of peak electrical power demand may be needed if multiple accessories are used at once [1307]. Typical engine power requirements for the air-conditioner, blower motor and the engine cooling fan are shown in Table 1 [1310].

Table 1
Typical engine power requirements for basic services at idle
AccessoryPower, hp
At 600 rpmAt 1200 rpm
Air-Conditioner, 32°C dayDraw down a hot cab7.57.5
Maintain a cool cab3.53.5
Blower Motor0.40.4
Engine Cooling FanOn/Off engaged1.55.0
On/Off disengaged00

In colder climates, engines are often idled—for instance during loading/unloading of cargo—to prevent potential start-up problems. Some truck drivers have developed a habit of always keeping a diesel engine operating.

Another significant source of diesel idle emissions can be railway locomotives. Unlike trucks, most locomotive engines do not use anti-freeze in their cooling systems. Thus, locomotives must idle their engines when the temperature drops below about 4°C (40°F) to prevent freezing of engine cooling water, thickening of engine oil and fuel and to maintain battery charge. At temperatures above 4°C, locomotives may idle to maintain a readily available engine, and/or to maintain comfortable temperatures inside the operator cab.

Motor coach buses are another vehicle category that can experience long periods of idling of their main propulsion engine. This is primarily to maintain a comfortable interior compartment for passengers (heat or air conditioning). While not as numerous as trucks, coaches have attracted attention because, due to their large interior compartment, maintaining a comfortable interior temperature requires substantially more idling time than the typical long-haul truck or personal passenger vehicle [1316].

Long-duration idling of truck, locomotive and other diesel engines can have several negative impacts on the environment and economy, as follows:

Contribution of Idling to Total Emissions

In North America, a large portion of onroad diesel fuel is used by Class 8 tractor trailers hauling goods. In 1995, the American Trucking Association estimated that trucks idle an average of 10 hours/day in the winter and 4.5 hours/day on non-winter days [1310]. Other estimates suggest maximum idle times of up to 8 hours per day [1308]. Data from vehicle engine control module (ECM) downloads found that long haul heavy heavy-duty trucks spend 40% of their time idling and 50% of their time in cruise mode [1311]. It is therefore informative to examine the case of idling long-haul highway trucks to estimate the proportion of total daily emissions and fuel consumption that can be attributed to idling from these vehicles.

The contribution of idle emissions to the total emissions from laden highway trucks can be estimated based on data from several CRC studies [1301][1302]. This estimate will assume an extreme case where a truck idles for 8 hours a day and spends 10 hours on the road cruising at highway speed. Table 2 outlines the results of this estimate based on the average of the emissions and fuel consumption for 36 heavy-duty trucks. Emission factors were estimated from the average values for tests carried out at idle with no accessory loads and on the cruise mode of the HHDDT cycle.

Table 2
Average estimated daily emissions and fuel consumption for laden highway truck
 Idling, 8 hoursCruising, 10 hours
CO, g2221850
NOx, g5677516
THC, g109314
PM, g20395
Fuel consumption, kg11.6197

The data in Table 2 could be used to estimate the relative contribution of idling to total emissions for the considered example. In a different approach, one can first estimate the relative idling contribution for each vehicle and then average these values. This allows a better estimate of the range of values for the idling contribution. Figure 1 shows the results of this estimate. While the averaged daily idling contribution is similar to the proportion that would be estimated from the averaged data of Table 2, the results are not necessarily identical.

Figure 1. Contribution of idling to total truck emissions

Range of contributions of idling to total daily emissions and fuel consumption for a truck idling for 8 hours a day. Error bars represent the average value ± one standard deviation.

For the extreme case of a highway truck that spends about 8 hours a day idling, about 6% of the daily fuel use could be attributed to idling. The contribution of all emissions from idling exceeds the contribution of fuel consumption. The contributions of CO and THC are especially disproportionate at 12% and 27%, respectively. These estimates assume no accessory loads and a low speed idle. If cab heat, air conditioning or a significant electrical load is required, the idling proportion will increase.

Two factors could account for the disproportionately high CO and HC emissions at idle. Diesel combustion at idle conditions has a high proportion of premixed combustion. This premixed burn is fuel rich and local fuel-air equivalence ratios can be as high as 4. Such rich conditions would produce significant amounts of CO. Also, over-lean conditions at the periphery of the premixed rich burn can lead to quenching of partially oxidized mixture or mixture escaping the combustion altogether.

PM emissions at engine idle can show a much larger number of particles than operation at load. These particles have also been reported to be much smaller (20 nm) than those at load (60 nm) [1768].