Emissions testing showing skewed results designed to invalid the benefits of H2 added to gasoline. The only way to get more unburned hydrocarbons and increased CO levels is to run a rich fuel mix. Naturally the poster of these "test results" says this proves HHO cannot increase MPG. Scams are not only present in he HHO community they also are done by the naysayers...

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ICE efficiency map

Here's an efficiency map for a motor vehicle courtesy of fueleconomy.gov

Listed below are losses described in detail. Notice that the Accessory category includes the A/C compressor, Power Steering Pump, Water pump, and the Alternator. An additional load on the system from electrolysis is really small in the scope of things.

Idling Losses – 17.2%

In urban driving, significant energy is lost to idling at stop lights or in traffic. Technologies such as integrated starter/generator (ISG) systems help reduce these losses by automatically turning the engine off when the vehicle comes to a stop and restarting it instantaneously when the accelerator is pressed.

Accessories – 2.2%

Air conditioning, power steering, windshield wipers, and other accessories use energy generated from the engine. Fuel economy improvements of up to 1% may be achievable with more efficient alternator systems and power steering pumps.

Engine Losses – 62.4%

In gasoline-powered vehicles, over 62% of the fuel's energy is lost in the internal combustion engine (ICE). ICE engines are very inefficient at converting the fuel's chemical energy to mechanical energy, losing energy to engine friction, pumping air into and out of the engine, and wasted heat.

Advanced engine technologies such as variable valve timing and lift, turbo charging, direct fuel injection, and cylinder deactivation can be used to reduce these losses.

In addition, diesels are about 30-35% more efficient than gasoline engines, and new advances in diesel technologies and fuels are making these vehicles more attractive.

Driveline Losses – 5.6%

Energy is lost in the transmission and other parts of the driveline. Technologies, such as automated manual transmission (AMT) and continuously, are being developed to reduce these losses.

Overcoming Inertia; Braking Losses– 5.8%

To move forward, a vehicle's drive train must provide enough energy to overcome the vehicle's inertia, which is directly related to its weight. The less a vehicle weighs, the less energy it takes to move it. Weight can be reduced by using lightweight materials and lighter-weight technologies (e.g., automated weigh less than conventional automatics).

In addition, any time you use your brakes, energy initially used to overcome inertia is lost.

But the addition of HHO to the system has the potential for gains greater than the losses associated with electrolysis because of H2 allows you to lean out the gasoline A/F ratio.