ignition

Spark Advance Control

The Spark Advance Control system maximizes engine efficiency by continuously adjusting spark advance timing to deliver peak combustion pressures when the piston reaches about 10' after TDC. Incorrect spark timing can have a significant effect on emission output and vehicle driveability. If ignition timing is excessively advanced during certain conditions, detonation will occur resulting in increased HC and NOx levels. Since NOx production is most predominant under loaded engine operating conditions, the spark advance system must ensure accurate ignition timing during these conditions. If ignition timing is incorrectly retarded, only partial combustion will take place resulting poor engine performance and increased emission levels.

Causes of Incorrect

On systems that use the ECM to compute ignition spark advance, there are only two conditions which are likely to cause spark timing to be incorrect; initial timing or a false input signal to the ECM.

The first step in troubleshooting emissions and driveability concerns should always include a quick check of initial ignition timing. Any error in initial timing will be reflected throughout the entire spark advance curve.

If engine load is miscalculated because of incorrect input signals, spark advance angle will not be appropriate for engine operating conditions. This will result in driveability and emission problems. Refer to course 850 for additional information on spark advance strategy.

The Effects of Fuel Octane

Toyota engines equipped with a knock detection system are very sensitive to fuel octane levels. Motor fuels with low octane ratings will cause the engine to detonate, which will in turn, cause the detonation retard system to retard timing. On some vehicles with advanced ECM operating strategies, an adaptive memory factor is used to track signals from the knock sensor. When detonation occurs frequently, the ECM relearns the basic spark advance curve, retarding spark throughout the entire engine operating range. This retarded spark curve will negatively effect engine performance and fuel economy under all driving conditions, even after a tank of higher octane fuel is purchased. The retarded spark curve will remain stored in the ECM keep alive memory until the engine is operated for a substantial amount of time on the higher octane fuel, or until the "keep alive memory" is cleared by removing power from the BATT terminal.

Purpose of Spark Advance Control Systems

The amount of spark advance needed by the engine varies depending on a number of different operating conditions. Generally, spark advance follows the following strategy:

•      spark advance increases with higher engine speeds for performance and fuel economy.

•      spark advance needs to decrease under heavy load conditions to avoid detonation.

They are many variables the system must consider when determining the proper spark lead time. Coolant temperature, fuel quality, and engine load are just a few of the many factors that can significantly impact ideal ignition time. The ECM determines proper spark timing by applying various input signals against a preprogrammed spark advance strategy or "map".

Fuel injected Toyota vehicles use either a mechanical or electronic spark advance control system. They are referred to as either conventional EFI ignition system (mechanical), Variable Advance Spark Timing (VAST) or Electronic Spark Advance (ESA).

From : Toyota Motor Sales, U.S.A., Inc. All Rights Reserved.

catalytic converter

Regardless of how perfect the engine is operating, there will always be some harmful byproducts of combustion. This is what necessitates the use of a Three-Way Catalytic (TWC) Converter. This device is located in-line with the exhaust system and is used to cause a desirable chemical reaction to take place in the exhaust flow.

Essentially, the catalytic converter is used to complete the oxidation process for hydrocarbon (HC) and carbon monoxide (CO), in addition to reducing oxides of nitrogen (NOx) back to simple nitrogen and carbon dioxide.

TWC Construction

Two different types of Three-Way Catalytic Converters have been used on fuel injected Toyota vehicles. Some early EFI vehicles used a pelletized TWC that was constructed of catalyst coated pellets tightly packed in a sealed shell, while later model vehicles are equipped with a monolith type TWC that uses a honeycomb shaped catalyst element. While both types operate similarly, the monolith design creates less exhaust backpressure, while providing ample surface area to efficiently convert feed gases. The Three-Way Catalyst, which is responsible for performing the actual feed gas conversion, is created by coating the internal converter substrate with the following key materials:

•      Platinum/Palladium; Oxidizing catalysts for HC and CO

•      Rhodium; Reducing catalyst for NOx

•      Cerium; Promotes oxygen storage to improve oxidation efficiency The diagram below shows the chemical reaction that takes place inside the converter.

TWC Operation

As engine exhaust gases flow through the converter passageways, they contact the coated surface which initiate the catalytic process. As exhaust and catalyst temperatures rise, the following reaction occurs:

•      Oxides of nitrogen ( NOx) are reduced into simple nitrogen (N2) and carbon dioxide (CO2)

•      Hydrocarbons (HC) and carbon monoxide (CO) are oxidized to create water (H2O) and carbon dioxide (CO2)

Catalyst operating efficiency is greatly affected by two factors; operating temperature and feed gas composition. The catalyst begins to operate at around 550' F.; however, efficient purification does not take place until the catalyst reaches at least 750' F. Also, the converter feed gasses (engine-out exhaust gases) must alternate rapidly between high CO content, to reduce NOx emissions, and high O2 content, to oxidize HC and CO emissions.

Effects of Closed Loop Control on TWC Operation

To ensure that the catalytic converter has the feed gas composition it needs, the closed loop control system is designed to rapidly alternate the air/fuel ratio slightly rich, then slightly lean of stoichiometry. By doing this, the carbon monoxide and oxygen content of the exhaust gas also alternates with the air/fuel ratio. In short, the converter works as follows:

• When the A/F ratio is leaner than stoichiometry, the oxygen content of the exhaust

stream rises and the carbon monoxide content falls. This provides a high efficiency operating environment for the oxidizing catalysts (platinum and palladium). During this

lean cycle, the catalyst (by using cerium) also stores excess oxygen which will be released to promote better oxidation during the rich cycle.

• When the A/F ratio is richer than stoichiometry, the carbon monoxide content of the exhaust rises and the oxygen content falls. This provides a high efficiency operating

environment for the reducing catalyst (rhodium). The oxidizing catalyst maintains its efficiency as stored oxygen is released.

As mentioned in the beginning of this section, precise closed loop control relies on accurate feedback information provided from the exhaust oxygen sensor. The sensor acts like a switch as the air/fuel ratio passes through stoichiometry. Closed loop fuel control effectively satisfies the three way catalyst's requirement for ample supplies of both carbon monoxide and oxygen. Generally speaking, if the closed loop control system is functioning normally, and fuel trim is relatively neutral, you can be assured that the air induction and fuel delivery sub-systems are also operating normally. If the closed loop control system is not working properly, the impact on catalytic converter efficiency, and ultimately emissions, can be significant.

Effects of Oxygen Sensor Degradation

Since the oxygen sensor is the heart of the closed loop control system, proper operation is critical to efficient emission control. There are several factors which can cause the oxygen sensor signal to degrade and they include the following:

•      Silicon contamination from chemical additives, some RTV sealers, and contaminated fuel.

•      Lead contamination can be found in certain additives and leaded motor fuels.

•      Carbon contamination is caused by excessive short trip driving and/or malfunctions resulting in an excessively rich mixture.

The effects of sensor degradation can range from a subtle shift in air/fuel ratio to a totally inoperative closed loop system. With respect to driveability and emissions diagnosis, a silicon contaminated sensor will cause the most trouble. When silicon burns in the combustion chamber, it causes a silicon dioxide glaze to form on the oxygen sensor. This glaze causes the sensor to become sluggish when switching from rich to lean, and in some cases, increases the sensor minimum voltage on the lean switch. This causes the fuel system to spend excessive time delivering a lean mixture.

From : Toyota Motor Sales, U.S.A., Inc. All Rights Reserved.