May 21, 2012

EPA Technical Memorandum

Subject: Energy Recovery Testing for GM Silverado 2- Mode Hybrid and other Hybrid Lightduty Vehicles

From: James Sanchez, Ben Ellies, U.S. Environmental Protection Agency

-----------------------------------------------------------------------------

Introduction

To provide additional data to inform the technology incentive credit program for full-size pickup 

trucks proposed for the EPA and NHTSA 2017-2025 light duty vehicle greenhouse gas and 

CAFE programs, EPA tested a full-size pickup truck hybrid along with several other hybrid

vehicles at the National Vehicle and Fuels Emission Laboratory (NVFEL). The results can be 

used to help inform the performance criteria for defining mild and strong full size pickup truck 

hybrids.

1

 The performance metric evaluated by the test program is based on the amount of 

energy that is absorbed by the battery compared to the kinetic energy that is available from the 

vehicle over a Cold and Hot Urban Dynamometer Driving Schedule (UDDS). The testing and

analysis of the data for a GM Silverado Hybrid are described in detail in this memo along with 

the summary results from the other hybrid vehicles tested at NVFEL over the past year2.

 

1

See 76 FR 74879 (Section I.B.4.e of the joint preamble) and Chapter 5.3 of the draft Joint 

Technical Support Document for the Joint Notice of Proposed Rulemaking for a detailed 

discussion of the proposal regarding the full-size pickup truck technology incentive credit, and 

the proposed test procedure and performance metric for distinguishing between mild and strong 

hybrid vehicles.

2

These additional hybrid vehicles (passenger cars, cross-overs, and sport-utility vehicles,) were 

tested for purposes other than measurements to inform the full-size pickup truck technology 

incentive credit, however, data was collected during those tests regarding current flow into the 

hybrid battery system which is informative and therefore presented here.
Vehicle Description

Table 1 – Description of vehicle tested.

Make GM

Model Silverado 2-Mode Hybrid

Year 2012

Beginning Odometer 

(miles) 4435

Test Weight (lbs) 6000

Target 2WD Dyno Set 4WD Dyno Set

A Coefficient (lbf) 51.63 30.93 7.39

B Coefficient (lbf/mph) 0.105 -0.1621 -0.2143

C Coefficient (lbf/mph^2) 0.034 0.03434 0.033

Fuel Certification Gasoline

Since this truck is the only full size pick-up truck strong hybrid commercially available, EPA 

requested a loan of this vehicle from General Motors for the purposes of characterizing a strong 

hybrid. In April 2012, General Motors provided this truck to EPA for testing. 
Test Plan

To prepare the vehicle the fuel tank was drained and filled to 60% of capacity. The 

dynamometer set coefficients were then derived with the dynamometer configuration in 2 and 4 

wheel drive mode. The vehicle was then driven over the UDDS cycle the day before the first set 

of tests were run. 

A current clamp was connected around the positive high voltage DC cable going from the power 

electronics to the nickel-metal hydride hybrid battery. The specific location was under the air 

filter box which is on the left side of the engine compartment. A Hioki 3193 meter was used to 

integrate and save the current data. The actual battery voltage was unable to be measured since 

there was no access to the electrical leads. The nominal battery voltage was used for all 

calculations. 

The vehicle was then tested over a cold UDDS, 10 minute soak and then a hot UDDS. The 

vehicle was tested once each day until there were 3 repeats with the dynamometer configured in 

2 and 4 wheel drive mode. Along with the high voltage DC battery current, dilute CO2, CO, 

NOx, THC, and CH4 were measured.

Data Analysis

Available Brake Energy

The theoretical maximum available brake energy over the schedule, Ebrake_max, was calculated 

using the vehicle’s equivalent test weight (ETW) and target coastdown coefficients A, B and C. 

The integration of the brake energy was done with a 10hz version of the UDDS schedule.

-1 2

-1

-1

-1

-1

-

-

1000

0

-

3600

i i

i i i

i i

i

i

i i i

i i

i i

V V M A B V C V V

t t

P

if P

P t t

E E

else

E E

Where:

i = a counter to indicate a point in time over the driving schedule in seconds. 

A = constant value representing the vehicle’s frictional load in N. 
B = coefficient representing load from drag and rolling resistance, which are a function of 

vehicle speed in N∙s/m. 

C = coefficient representing aerodynamic effects, which are a function of vehicle speed squared

in N∙s2/m2.

V = linear speed of the vehicle (in the case of the Silverado) or schedule (for the other vehicles 

tested), in mph or m/s.

M = mass of vehicle in kg. 

t = elapsed time in the driving schedule.

P = road load power in kW.

E = brake energy in kW-hr

Per the above equations, the final value of Ei (at the last time point in the trace) represents 

Ebrake_max.

Note that the A, B and C coefficients are typically reported in English units (lb, lb/mph and 

lb/mph2), and as such must first be converted to their metric equivalents. The expression 

(A+Bv+Cv2), combined with vehicle speed in mph, can be converted to N by multiplying by 

4.448.

Electrical Energy to Battery

The electrical energy to the battery was measured with the Hoiki 3193 meter with the current 

clamp between the battery and the power electronics. This location it consistent with where the 

current clamp is installed for the SAE J1711 state-of-charge (SOC) check. The amp-hours (Ahr) to the battery was integrated with the Hioki meter. The notation used was: current going to 

the battery is negative and current out of the battery is positive.

-1

-1

-1

0

-

3600

i

i i i

i i

i i

if I

I t t

Ahr Ahr

else

Ahr Ahr
The current data was also integrated only during the braking portions of the cycle. 

-1

-1

-1

0 & 0

-

3600

i i

i i i

i i

i i

if I P

I t t

Ahr Ahr

else

Ahr Ahr

I = current in and out of battery in ampere.

Ahr = electrical charge transfer in and out of battery in ampere-hour.

Erecovered is calculated by multiplying the nominal voltage (as reported by the manufacturer) to 

the final value of Ahri (at the end of the test).

Results for 2-mode Silverado Pickup Truck

The emissions results and battery SOC change are summarized in Table 2. Since there wasn’t a 

significant offset between tests with the dynamometer in 2 and 4 wheel drive mode the mean, 

standard deviation, coefficient of variance (COV) were calculated using all of the tests. The test 

to test variability of CO2 emission rate is correlated with battery SOC change over the test. The 

tests with a larger amount of energy flowing out of the battery had lower CO2 emissions than the 

tests with less energy coming from the battery.
Table 2 – Summary of gaseous emissions and battery SOC change for 2-mode Silverado Pickup 

Truck Hybrid.

Test # Dyno Mode CO2 HC-FID CO NOx CH4 NMHC Delta SOC

g/mile

A-hr

(+ is energy out of 

battery)

1 2 wheel drive 502.7 0.0730 1.152 0.0198 0.0160 0.0566 0.060

2 2 wheel drive 440.0 0.0445 1.261 0.0222 0.0165 0.0277 0.238

3 2 wheel drive 444.2 0.0484 1.390 0.0227 0.0184 0.0299 0.189

1 4 wheel drive 459.2 0.0499 1.565 0.0227 0.0199 0.0299 0.146

2 4 wheel drive 471.8 0.0841 1.822 0.0247 0.0231 0.0605 0.068

3 4 wheel drive 474.3 0.0883 1.807 0.0234 0.0238 0.0641 0.124

Mean 465.36 0.065 1.500 0.023 0.020 0.045 0.138

Standard 

Deviation 23.00 0.019 0.280 0.002 0.003 0.017 0.069

COV 5% 30% 19% 7% 17% 39% 50%
Energy Results for 2-mode Silverado Pickup Truck and Additional Vehicles

Table 3 is a summary of the available brake energy (Ebrake_max), total energy to battery (Erecovered) 

and energy to the battery during braking for the Silverado and additional hybrid vehicles tested at 

the NVFEL. Except for the Silverado, Hioki data (originally recorded at 1 Hz) for all vehicles 

was post-processed from earlier hybrid vehicle emissions tests, as documented in the attached 

Excel spreadsheet. The exact measurement location for each test vehicle was not known, 

although it was in accordance with the standard state-of-charge balance confirmation protocol 

used for all hybrid vehicle certification testing.

Table 3 – Summary of available brake energy and energy to the battery.

Theoretical Total Charging

Decel-only 

Charging

Ebrake_max Erecovered % of Test data % of

type kWh kWh Ebrake_max kWh Ebrake_max

Lacrosse Mild -1.666 0.34 20% 0.15 9%

Civic H IMA -1.234 0.63 51% 0.32 26%

Prius V Strong -1.430 1.19 84% 0.54 38%

Camry Strong -1.519 1.58 104% 0.81 53%

Highlander Strong -2.048 1.90 93% 0.94 46%

CT 200h Strong -1.325 1.16 87% 0.56 43%

Silverado 2-mode Strong -2.287 1.67 73% 0.97 42% 6-test avg

Attachment: Braking energy calculator (4-bag) 052112.xlsx