UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY

2565 PLYMOUTH ROAD

ANN ARBOR, MI 48105-2498

 OFFICE OF

 AIR AND RADIATION

May 8, 2015

MEMORANDUM

SUBJECT: Analysis of Possible Vocational Vehicle Standards Based on Alternative Idle Cycle 

Weightings

FROM: Lauren Steele, Environmental Engineer, OTAQ/ASD

TO: Greenhouse Gas Emissions Standards and Fuel Efficiency Standards for Medium- and 

Heavy-Duty Engines and Vehicles - Phase 2 - Docket EPA-HQ-OAR-2014-0827

Introduction

This memorandum presents estimated emission reductions and fuel consumption savings based 

on alternate weightings of the proposed GEM idle cycle for vocational vehicles. Depending on 

additional information that may become available during the comment period, it is possible that the 

agencies may change the weighting of the idle cycle in the composite GEM cycles for vocational 

vehicles from the proposed values to values that better reflect real world vocational vehicle operation. 

This memo explores the effects of such change on technology effectiveness and vehicle performance, as 

simulated through GEM.

In GEM the emissions and fuel consumed during the idle cycle are time-based (grams or gallons

per hour), and the distance-based emissions and fuel consumed while moving (grams or gallons per tonmile) are calculated separately. Because the combined weightings of the three distance-based cycles plus 

the idle cycle total 100 percent, any increase in the weighting of idle cycle emissions causes a 

proportional decrease in weighting of the emissions while moving. This means that the degree of 

weighting of the idle cycle influences the relative importance of idle reduction technology compared 

with technologies that reduce emissions while driving. For example, an increase in idle cycle weighting 

would lead to a smaller relative importance of technologies that reduce engine emissions and rolling 

resistance.

Range of Idle Cycle Weightings

According to preliminary work by the National Renewable Energy Laboratory (NREL), logged 

activity of vehicles in the NREL database indicates the overall percent of idle time ranges between eight 

and 50 percent, with most vehicles logging idle time between 18 and 33 percent. Figure 1 shows an 

example summary of some NREL idle operation data for vocational vehicles.
2

Figure 1: Percent Idle Time per Day per Vehicle. Source: NREL

Using the GEM tool, the agencies have estimated the effectiveness of workday idle reduction 

technology (neutral idle and stop-start) across a wide range of idle cycle weightings. Figures 2 and 3 

present the estimated effectiveness of neutral idle and stop-start, respectively, for idle cycle weightings 

up to 50 percent. The proposal case weights all weight classes of Regional vehicles at 10 percent idle, 

Multi-purpose vehicles at 15 percent idle, and Urban vehicles at 20 percent idle, regardless of engine 

type. For each curve in the graph, the percent reduction from the GEM baseline is calculated from a 

baseline that uses the same weighting of idle as for the idle reduction technology case. For example, at 

30 percent idle cycle weighting, a MHD Urban vehicle with neutral idle would have a GEM score 5.5 

percent lower than a baseline vehicle with that same idle cycle weighting.

Figure 2: Change in Effectiveness of Neutral Idle Workday Idle Reduction Technology with Cycle Weighting

0%

2%

4%

6%

8%

10%

12%

Percent Reduction fro

m

GEM

Baseline

Relative Percent Weighting of Idle Cycle (%)

Sweep of Possible Neutral Idle Reductions

HH Regional

HH Multipurpose

HH Urban

MH Regional

MH Multipurpose

MH Urban

LH Regional

LH Multipurpose

LH Urban
3

Figure 3: Change in Effectiveness of Stop-Start Workday Idle Reduction Technology with Cycle Weighting

Figure 4 presents the relative contribution to GEM baseline values (no added technology and all 

vehicles idling at CITT emission rates) as the weighting of the idle cycle changes from zero to 50 

percent. The emission rates in grams of CO2 per ton-mile decrease with increasing idle weighting due to 

the relatively lesser weighting of emissions over the ARB transient, 55 mph cruise and 65 mph cruise

cycles. In the example of the HHD Urban vehicle (dark green bars on the graph), the contribution of the 

ARB Transient cycle is about 40 percent of the total mass of CO2 in g/ton-mi when the idle cycle is 

weighted zero (first cluster of bars). In the case of a 30 percent idle cycle weighting, the transient 

contribution from this same HHD Urban vehicle would be about 36 percent of the total mass of CO2 in 

g/ton-mi.

0%

5%

10%

15%

20%

25%

Percent Reduction fro

m

GEM

Baseline

Relative Percent Weighting of Idle Cycle (%)

Sweep of Possible Stop-Start Idle Reductions

HH Regional

HH Multipurpose

HH Urban

MH Regional

MH Multipurpose

MH Urban

LH Regional

LH Multipurpose

LH Urban
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY

2565 PLYMOUTH ROAD

ANN ARBOR, MI 48105-2498

 OFFICE OF

 AIR AND RADIATION

Figure 4: Change in Baseline Performance with Idle Cycle Weighting

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

Transient

55 mph65 mph

Idle

Transient

55 mph65 mph

Idle

Transient

55 mph65 mph

Idle

Transient

55 mph65 mph

Idle

Transient

55 mph65 mph

Idle

Transient

55 mph65 mph

Idle

Percent C

ontribution to

GEM

C

o

mposite Weighted Mass ofC

O2

g/ton-mi

Test Cycles with Idle Cycle Weightings from Zero to 50%

HH Regional HH Multipurpose HH Urban MH Regional MH Multipurpose MH Urban LH Regional LH Multipurpose LH Urban

Proposed

10-15-20

20% Idle 30% Idle

40% Idle 50% Idle

Zero Idle