AEI’s business includes both government and private industry customers. A partial listing of clients is shown below:
| Stant | South West Research Institute | Senior Flexonics |
| Renault | National Renewable Energy Laboratory (NREL) | Faurecia Emissions Control Technologies |
| ITT Aerospace Command | U.S. Environmental Protection Agency (EPA) | Electro Motive Diesel |
| Coltec, Inc. | U.S. Special Operations Command (SOCOM) | Aramco Services Co. |
| Scott-Gibbon Ltd. | Cummins, Inc. | Fire Chariot, LLC |
| Allison Transmission | Naval Undersea Warfare Center | University of California, Riverside |
| U.S. Dept. of Defense (DOD) | Cosworth Racing | TATA Cummins, LTD. |
| Applied Laboratories | U.S. Department of Energy (DOE) | Kirloskar Oil Engine LTD |
| MAN Nutzfahrzeuge AG | Industrial Power Alliance | IAV GMBH |
| Ford Motor Co. | Perkins Engine Co. | BMW Motoren |
| CARB | PARU Engineering Co. | GE Global Research |
| Martin Marietta Materials, Inc. | Donaldson Company, Inc. | Affiliated Constitution Services |
| John Deere | Fleetguard | Liebherr |
| Borg Warner | IVECO Trucks Limited | AVL |
| Mack Trucks, Inc. | California Air Resources Board (CARB) | FPT Industrial |
| Detroit Diesel Corp. | NOxTech Inc. | Mainco |
| Thermal Dynamics | Eaton Corp. | MSX International |
| DAF Trucks | American Chemical Council (ACC) | Navistar |
| General Motors Corporation | Emissions Technology | Robert Bosch Corp |
| Kukje Machinery Co. Ltd | Engineering Test Services | Caterpillar, Inc. |
| PSA Peugeot Citroën | Dow Chemical Co. | |
| Cummins East Asia R & D Ltd. | Volvo Powertrain Corp | |
| International Truck and Engine | John Deere Usine de Saran | |
| Ricardo UK Ltd. | Mitsubishi Heavy Industries Ltd. | |
| Daihatsu Diesel Mfg Co Ltd. | Daimler Benz | |
| Jiangling Motors | Emcon Technologies | |
| Alcoa | Karl Schmidt Unisia | |
| Tenneco | Houston Air Research Center |