Horsepower as Well as Clean Emissions

The Offenders

We are concerned with hydrocarbons, carbon monoxide, carbon dioxide, oxygen, and oxides of nitrogen. Each is the result of inefficient combustion.

Hydrocarbons (HC)-Gasoline consists of hydrogen and carbon atoms, thus levels of hydrocarbon represent unburned fuel in the exhaust. Identified in parts per million (PPM), this is the emission that makes your eyes tear from the exhaust of a poor-running car or an engine with a large cam. Mistakenly thought of as a rich air/fuel (A/F) mixture, HC is known as a misfire meter. HC levels escalate when the engine is not experiencing complete and efficient combustion. Residual HC is created by ignition problems, excessively lean A/F mixtures, camshaft design, and excessive crevice volumes or carbon deposits in the combustion chamber.

Carbon Monoxide (CO)-Measured in percent, CO represents partially burned fuel created by a lack of necessary oxygen to support combustion. Simply put, whenever CO readings are high, the A/F mixture is overly rich. A failed CO test indicates a plugged air filter, poorly adjusted carburetor, high float level, high fuel pressure, dirty injectors or degraded oxygen sensor, or fuel dilution in the crankcase.

Carbon Dioxide (CO2)-As a by-product of the bonding of the carbon and oxygen molecules during combustion, C02 demonstrates how efficient the engine is running. It is measured in percent.

Oxygen (02)-Defined as a colorless, odorless gas, this is the most common element known to man. During an emissions test, high levels in the exhaust represent additional oxygen molecules that do not have a carbon element to bond with. Oxygen readings increase if the mixture is excessively lean and also during scenarios that would normally promote high HC levels. If the fuel is not being burned, neither is the oxygen. Usually inversely proportioned to C02, 02 is measured in percent.

Oxides of Nitrogen (NOx)– Measured in PPM, NOx is present during all phases of combustion but escalates dramatically when leading-edge flame-front temperatures are in excess of 2,500 degrees F. Requiring heat, cylinder pressure, and exposure time to be produced, detonation is a leading source of this pollutant. Exhaust gas recirculation (EGR) valves are used to cool combustion chamber temperatures by introducing inert exhaust gases. It may be hard to grasp how hot exhaust can cool a combustion chamber, but it actually functions by consuming space in the bore, thus limiting cylinder fill rates with a combustible mixture. Since NOx production is highest under part throttle light load, when the A/F mixture is the leanest, NOx became the impetus to mandate the I/M 240 schedule.

The Catalyst: Friend or Foe?

An emissions-control device in the exhaust tract is an attempt to correct what could not be controlled in the combustion chamber. Simply, a catalyst speeds up a chemical reaction without being consumed itself. In effect, the catalyst “scrubs” the exhaust. Early catalysts were filled with pellets made of 70-percent platinum and 30-percent palladium. The pellets were eventually replaced with a ceramic monolith that combined a larger surface area with smaller exterior dimensions. Not all catalysts are effective on all emissions. Application-specific catalysts are designed to neutralize certain elements. Oxidation catalysts convert CO and HC to C02 and H20, while reduction catalysts treat NOx by converting it back to nitrogen and oxygen. Dual-bed designs are a melding of two different catalysts that affect NOx in the first section and CO and HC in the rear section. Three-way catalytic converters simultaneously scrub all three exhaust gases in one unit.

For a catalyst to function properly, it must attain an operating temperature of 1,000 degrees F, but many driving scenarios produce temperatures of 1,400-1,500 degrees F or more. When the conversion process begins, the converter is experiencing the light-off mode. Converter efficiency is rated in percentage of conversion, with a minimum of 85 percent accepted for (1996-99) OBD-II vehicles. Due to the effects of thermal inertia, which requires a body to become saturated with heat before being able to transfer it, catalysts are placed as near to the exhaust ports as possible. Though definitely a flow restriction, a properly sized and placed catalyst costs only a few horsepower but has a dramatic effect on exhaust emissions with unleaded fuel. Contrary to popular belief, leaded fuels do not plug the converter but rather coat the catalytic material and insulate it from the act of chemical conversion.

Designing a Friendly High-Output Engine

Basically, we’re dealing with combustion efficiency and the gas exchange process, and the most challenging emissions to deal with are HC and NOx. These properties are inextricably linked to the means normally associated with increasing engine output-camshafts with overlap and high compression ratios. Carbon monoxide can be controlled with A/F ratio adjustments, while 02 is used as a diagnostic gas. Carbon dioxide is tested to determine if the vehicle has an exhaust leak.

When controlling HC production, crevice volume becomes highly relevant. The crevice is an area in the
combustion chamber that the flame front cannot access, but one where the A/F mixture will. The largest contributor to crevice volume is the area created by the piston and piston rings in relation to the cylinder wall. Other crevice areas are represented by the threads around the spark plug, the space around the center electrode, the space between the heads of the intake and exhaust valves and the cylinder head, as well as the head gasket cutout. As the cylinder pressure rises during compression, the mixture is forced into these regions. During combustion, cylinder pressure rises even more, forcing more HC into the crevices. Since the flame travel cannot access these areas, the A/F mixture waits in the crevices until cylinder pressure is lower than the crevice gas pressure and flows out of the exhaust. For this reason, the trend in current production engines is to move the top ring as close to the crown of the piston as possible, thus curtailing the major contributor to crevice volume. In an engine that is experiencing proper combustion, it has been determined that the ring package is responsible for 80 percent of the hydrocarbon production, the head-gasket crevice 13 percent, and the spark plug threads 5 percent. All other sources contributed the rest. Besides being a major producer of HC emissions, crevice regions waste fuel, which is being used as filler and not to add energy to the expansion cycle. How picky does it get? The OE industry has determined that the direction arrow stamped on the piston top was contributing to crevice area-so it was replaced with a mark inside the piston.

A camshaft with excessive overlap reduces low-rpm engine efficiency, which in turn promotes poor burn rates and results in high HC output. Overlap is not a completely bad thing. It can be used as an internal EGR function to pull spent gases from the exhaust port. A delicate balance of overlap is ideal but beyond the realm of the enthusiast. It will remain the domain of the cam companies to acknowledge the emissions issues. Since our success is linked directly to a systems approach, if crevice volumes are summarily reduced, then the engine would be more forgiving of overlap since HC emissions aren’t being stored.

Oxides of nitrogen output are the most difficult to balance. Although we need compression for thermal efficiency, higher cylinder pressure is what produces NOx. The easiest method for controlling NOx is a quick-burn combustion chamber. Temperature, pressure, and exposure are necessary for the promotion of NOx, but if we can speed up burn rates, we can limit exposure time.

In the ’70s, the domestic auto industry failed to realize that old, slow-burning combustion chambers would not work well enough to satisfy NOx standards, and it drastically reduced compression ratios instead. Rather than redesigning the combustion chamber, it took compression out and added nearly 30-percent EGR dilution Although EGR is an effective, inexpensive. means for controlling NOx, it disturbs burn cycles in the cylinder. The less EGR dilution, the better driveability will be. EGR is administered during part throttle light-load condition, so it will not affect idle quality or WOT performance. However, all of this is moot if the engine enters abnormal combustion known as detonation. NOx production goes off the scale during detonation due to the extreme temperature and pressure rise that an uncontrolled release of the end gas energy produces.

Many after market cylinder-head companies mimic the combustion-chamber technology of the latest offerings from Detroit. Quick-burn rates, internal charge acceleration by the means of squish pads, and external mixture motion from the swirl of the incoming charge will produce power and pay dividends toward compliance.

Putting It to the Test

The pending I/M 240 test got its name from the 240-second sampling time with the vehicle situated on a chassis dynamometer to simulate load. The engine runs at idle, is accelerated, and then allowed to idle again. The tester then averages the results.

To certify a vehicle for sale in the United States, the EPA mandates Federal Test Procedure-75 (FTP-75). Divided into three segments, the total test time is 4,500 seconds (75 minutes), an it is performed on an extremely sophisticated 48-inch roller chassis dyno designed to simulate morning commuter traffic in Los Angeles. Since 80-85 percent of engine emissions occurs during the first two minutes of operation, an independent cold-start test is also performed with the vehicle placed in a room and chilled to 20 degrees F.

In another procedure called the SHED, the vehicle is placed in a test cell where the HCs emitted while the engine is not running are measured for 24 hours. While state I/M programs measure readings in either percent or PPM, the FTP cycle weighs the tailpipe output and records grams of pollutants per mile of operation. For this reason, smaller-displacement engines are easier to certify since they displace less air, and even if they are dirtier, they’ll exhibit less recorded grams/mile. Some states will initially adopt a shorter version of I/M 240 called 5015. The difference is that it calculates 50 percent of the load while accelerating to 15 miles per hour.

Tuning Tips

An exhaust-gas analyzer is essential. The new I/M laws have made previous emissions analyzers invalid for the inspection procedure, but they are still excellent and affordable tuning devices for hot rodders. Units that had cost $5,000-$10,000 new are now in the $500 range. It is essential to realize that the catalyst functions effectively only within 2 percent of 14.7:1 rich or lean (an A/F ratio of 14.40-14.99). Wander beyond these values and the converter does little or nothing to clean the exhaust.

Carburetor Adjustment-Using an exhaust analyzer, obtain the best emissions readings by fully seating both mixture screws, and then turn each one out the same number of rotations before beginning the adjustments. You are shooting for the lowest CO reading with the highest C02 reading. Achieve this, and the engine will experience nearly perfect combustion. Remember, if you go too lean, HC will rise since there will be insufficient fuel in the combustion chamber for the flame front to propagate. Consequently, the flame goes out, and the left over fuel elevates HC emissions. Early power-valve actuation will raise the light-load CO readings. With a carburetor, the CO reading should be highest at idle speed since the throttle is closed, as rpm increases, it should reduce. If your readings go substantially richer than idle, check the float level, power-valve setting, and the air bleeds.