Charge Air After-cooling

Transcription

1 385 Charge Air After-cooling Introduction Air pressurized by turbochargers increases temperature which negatively effects engine durability, performance and emissions. After-cooling, inter-cooling or charge air cooling are interchangeable terms describing the system responsible for removing excess heat from the air charging the cylinders. Charge air cooling is an important technology for reducing N0x emission which also has a number of other benefits. Fuel economy, power output and the maximum injection rates an engine can sustain are improved through charge air cooling. This chapter not only examines the operating principles and benefits of charge air cooling but describes diagnostics, testing and servicing of charge air coolers. Learning Outcomes Identify charge air cooling system components Define and explain terminology associated with charge air cooling Define and explain the purpose and operating principles of charge air cooling systems Describe the functions, construction, types, styles and applications of charge air cooling systems Describe and explain methods for performing inspections and diagnostic procedures on charge air cooling systems Recommend maintenance or repairs of diesel engine charge air cooling systems. Key terms Jacket water cooling (JWAC) Charge air cooling (CAC) Air to air after cooling (ATAAC) Heat exchanger Temperature differential test Pressure drop test Inter-stage Cooling What is Aftercooling? The terms after-cooling and inter-cooling are used interchangeably by various manufacturers to describe cooling of intake airflow from the turbocharger outlet. Ambient (outside) air or engine coolant passing though heat exchangers are the mechanisms for exchanging heat contained in turbo-pressurized intake air. A chassis mounted air-to-air aftercooler (ATAAC) may be called a charge air cooler (CAC). If engine coolant is used to cool intake air it is referred to as jacket water aftercooling (JWAC).

2 386 Charge air cooling involves removing heat from the air pressurized by the turbocharger. Hot air from the turbocharger enters the aftercooler where heat is released to the atmosphere. Cooler air leaves the heat exchanger and enters the intake manifold. The charge air cooler is located between the turbocharger and the intake manifold. Why is aftercooling used? On turbocharged engines, air intake temperatures rise dramatically with boost pressure. High intake air temperatures in turn negatively affect emissions, engine durability and performance. The loss of air density and increased cylinder temperatures accompanying higher intake air temperatures are the main factors casuing these problems. Since pressurizing a gas increases its temperature, intake air temperature is raised proportional to its compressed pressure. Additionally, when gas temperatures increase, molecules spread farther apart and the gas loses density. This means a cubic foot of air, when is heated, contains fewer oxygen molecules and weighs less. The relationship between temperature and pressure can be predicted using mathematical equations. Consider the following examples: If intake or under hood intake air temperature is 75 º F at atmospheric pressure sea level) the turbo outlet temperature will be 285 º F at 20-psi boost pressure. If the inlet air temperature to the turbocharger climbs to 25 degrees to 100ºF, the discharge temperature will be 320 º F. 30-psi boost pressure produces 358ºF outlet temperature with a 75ºF intake temperature and 396 º F at 100 º F air inlet temperature.

3 387 A 250-HP engine will develop only 240-HP if the air temperature is 130F (54C) using the same quantity of fuel. Hot air from the turbocharger enters the aftercooler where heat is released to the atmosphere. Cooler air leaves the heat exchanger and enters the intake manifold. Problems with High Intake Air Temperatures 1. Low air density High cylinder-charging air temperatures results in lower air density with fewer oxygen molecules available for combustion resulting in poorer combustion quality. As air molecules spread farther apart, less oxygen is available to support combustion and the air mass available to cool cylinder components drops too. For every 10º F temperature increase above 60ºF the changes to air density and oxygen content results in a power loss. Power loss is 1% for every 10F above 90F. Without a cool air mass travelling across the piston crown from the intake to the exhaust valve during the valve overlap period, the piston crown and exhaust valve become very hot. The loss of air s cooling effect results in higher cylinder heat loads and shorter engine life. 2. Higher Combustion Temperatures Increases in the intake charge temperature result in significantly higher combustion and exhaust temperatures. This exhaust temperature increase is disproportional to the air inlet temperature increase with small increases in air inlet temperature often producing even larger changes to exhaust temperature. Generally a three to one ratio exists with every

4 388 one degree-f increase in charge air temperature producing a three degree increase in exhaust temperature. However, many factors including fuel rates, excess air ratio, air density, compression ratio and others affect this relationship. Exhaust temperatures should never exceed 1250ºF-1300ºF or damage to valves and pistons occurs. Very hot exhaust gases will also cause the thin outer edges of the turbocharger turbine to glow and eventually melt. If thermal loading of the cylinders is reduced through charge air cooling, larger quantities of fuel can be injected to develop more power from the same engine displacement. This piston shows evidence of excessive heat loading of the piston crown caused by excessive combustion temperature and pressure. This type of failure is common in engines modified for higher injection rates and advanced injection timing. A restricted air intake or externally restricted charge air cooler will cause these cracks too. Note the cracks are vertical and travel from the edge of the combustion chamber bowl. 3. Higher N0x emissions Increases to charge air temperatures result in higher combustion chamber temperatures which in turn increases the production of N0x emissions. Normally, nitrogen, which makes up approximately 77% of the composition of air, remains inert and uninvolved in combustion processes. At temperatures above approximately 2,500F and because of high cylinder pressures, nitrogen will react with oxygen to produce nitrogen oxygen compounds collectively known as N0x. Beginning in 1988, increasingly strict emission standards for N0x were introduced for diesel engines. The cooling of intake air which drops combustion temperatures goes a long way to minimizing the formation of these emissions and the intercooler is an important emission control device. Emission Tip Charge Air Cooling & Emissions 1988 marked the first year for legislated reductions to diesel emissions. Prior to this, diesels only needed to meet a standard for exhaust opacity during a throttle snap test. N0x emissions dropped from approximately 11-grams/Brake HP/Hr for heavy-duty on-highway diesels to 6 grams in During this time almost every medium and heavy-duty diesel vehicle began to be equipped with charge air cooling to bring N0x to these levels. Without adequate cooling of intake air combustion temperatures rise producing exponential increases in N0x emissions.

5 389 Techtip Leaking Charge Air Coolers Charge air coolers are large air handling components and vulnerable to damage. Leaking and cracked intercoolers are often a source of low power complaints. Cracks can easily develop in these devices due to constant thermal cycling from hot to cold. When investigating low-power complaints it is a good practice to inspect and test the intercooler for leaks as a matter of course. Air handling system technology, including charge air cooling has provided substantial benefits to emission reduction. Benefits of Aftercooling Cooling the intake air charge to the cylinders accomplishes the following important benefits 1. Increases air density, and oxygen content for improved combustion quality. More oxygen in the cylinders means better contact between fuel and oxygen for efficient clean combustion. 2. Intercoolers provide additional air mass for cooling of valves and pistons. Denser air means the air is heavier and can remove more heat from valves and pistons during valve overlap. Adding additional air mass to the cylinders helps dilute the heat produced during combustion for lower cylinder temperatures. 3. Denser cooler air produces up to 5% fuel economy improvement due to improved combustion qualities. The simple addition of an intercooler does not produce more power - that requires more fuel and air. However, to the extent cooling of intake air improves combustion quality, power increases are realized. 4. Intercoolers allow the addition of more fuel to cylinders to provide higher power output per cubic inch of cylinder displacement. Since the addition of extra fuel can push the thermal loads of engines to their maximum threshold, cooling charge air can reduce

6 390 combustion and exhaust temperatures. This makes it possible to gain addition power while minimizing the risk of engine damage using higher injection rates. 5. Intercoolers reduce N0x emissions by lowering peak cylinder temperatures and pressures through cooling the charge air. Note the addition of a CAC and larger exhaust to this medium duty diesel engine has only a marginal power improvement. Engines with charge air coolers usually have more power but it is a misconception to conclude that charge air cooling increases power output. Cooling air allows for delivery of more fuel because CAC lowers cylinder temperatures. Engines can operate with higher power output without excessive thermal cylinder loads using CAC. Charge air cooling and other air components used to handle air intake.

7 391 Types of Aftercooling Jacket Water Aftercooling (JWAC) This type of charge air cooling lowers intake temperatures by passing the boost air through water type heat exchanger. A JWAC is capable of lowering the turbocharger boost air temperatures from a temperature of about 300F down to approximately 200F. Older mechanically governed engines often used a JWAC positioned inside the intake manifold. The advantage for a mechanically governed fuel system is the injection timing can be calibrated around a relatively constant air inlet temperature. This type of cooling is still a popular on marine diesel engines. Today, JWAC are used in series turbocharging applications where very high boost pressures produce even hotter intake air temperatures. Air intake temperatures are first lowered by a JWAC before passing through an air to air cooler. This JWAC located inside the intake manifold of a mechanically governed L-10 uses engine coolant to lower intake air temperatures. The outlet of the turbocharger is connected to the intake manifold with a tube. The 2010 Maxforce 11, 13 and 15L engines use a JWAC for interstage cooling. The JWAC is identified with coolant lines. After passing though the first stage turbocharger the JWAC cools air before engineering the second stage turbocharger. A second JWAC is located on top of the engine which cools the second stage boosted air before passing though the air to air cooler and into the engine.

8 392 Location of the two JWAC s used on Maxforce 13 and 15L engines. Caterpillar uses a JWAC on its C- 13 and 15 L ACERT engines with series turbocharging. Prior to entering the air to air cooler, the JWAC lowers intake air temperature after passing though the low-pressure turbocharger. Techtip Servicing JWAC Cooling Systems Whenever the cooling system of an engine using a JWAC is serviced it is important to bleed air from the cooler. The cooler is a high point in the cooling system and air can be trapped in the cooler preventing proper functioning. Series turbocharged engines using JWAC s transfer more heat to the cooling system and therefore require more vigilant maintenance of the cooling system. Air to Air aftercooling The most popular method of cooling intake air is to use an air to air type heat exchanger. By moving the heated charge air through an air cooled heat exchanger, the temperature can be dropped from over 300F to between 100 and 110F with an outside temperature of 75F. The exact temperature reduction depends on what the ambient air temperature and the volume of air flow across the cooler core. A properly functioning intercooler has a maximum temperature differential of 50F between air intake temperature and outside air

9 393 at 30-MPH airflow. Most coolers have an efficiency of approximately 80% which means they reduce intake temperatures to no more than 30 or 40F above ambient outside temperatures. Air to air after cooling require relatively high air flow across the cooler to remove heat. Therefore winter fronts should not be used with ATAAC. If a winter front is used it should never be closed completely at least 20% air flow must remain. Most engine manufacturers using electronic controls monitor air intake manifold temperatures as a part of the engine protection system. High intake temperatures will cause the engine to de-rate power or even shut-down. A/C Condenser Air To Air Cooler Fuel Cooler Radiator Transmission Cooler Today s diesel have substantial amounts of cooling. The charge air cooler is a down flow type. Charge Air Cooler An air to air charge air cooler is mounted over or beside the radiator to take advantage of maximum airflow when the vehicle is moving. Radiator

10 394 Construction and Operation Most ATAAC are constructed of aluminium for maximum heat transfer and strength. In order to achieve optimum cooling through the inlet and outlets of the cooler may have different diameters. This is necessary to slow the airflow through the cooler to maximize heat transfer. Aluminum side tanks and an aluminum core of a typical air to air aftercooler (ATAAC). As the temperature in the cooler drops, a corresponding pressure drop occurs too. A cooler pressure drop from side to side should not typically exceed more than 2-psi. If the pressure drop is greater than this, the cooler should be check for internal restrictions and leaks. CAC are a part of the emission control system and must be maintained like any other emission control device to ensure a vehicle is emission compliant. Intercooler Mounting Since coolers are often made from aluminium they expand and contract when heated and cooled. For this reason the mounting system of a cooler is designed to allow for thermal cycling. For example, washers on both sides of mounting bolts may include a spring type washer along with nylon like washers which allow the cooling to slide. Special care needs to be given to manufactures mounting instructions or damage to the cooler will result.

11 395 The Subaru diesel intercooler shown here has elongated slots at its mounting locations to allow for expansion and contraction of the cooler. Mounting hardware cannot be too tight and prevent movement. Testing and Servicing ATAAC Charge air coolers can fail for a number of reasons which can lead to an air inlet restriction, excessive emissions, high exhaust and cylinder temperatures. Low power complaints can be caused by internal or external restrictions to the intercooler. Electronic engines will often derate the power output if high intake manifold temperatures are measured. These conditions can lead to severe damage to the engine. Visual checks of intercoolers include checking for various cracks in cooler tubes and the joint between the tanks and tubes. It is critical to inspect for external restrictions in front and behind the intercoolers. ATAAC Service Conditions Leakage Since the cooler is subjected to large temperature differentials as air cools moving from one side of the cooler to another. Thermal cycling can stress the cooler until it cracks. The stress is compounded since the aluminium has a much higher expansion co-efficient than other metals. It is common then to find cracks in areas of the cooler where inlet temperatures are highest and at the joint between the inlet tank and cooler tubes.

12 396 Replacement is not always the expected service recommendation for this condition since the size of the some cracks does not justify cooler replacement. Pressure testing Pressure testing the cooler to determine the magnitude of air volume lost is the most important criteria for justifying cooler replacement. Small cracks do not warrant replacement since dirt ingestion into the engine through a crack is not likely when a cooler is pressurized. Additionally, if the engine is using excess air for combustion, some leakage is permissible before the air loss affects the engine performance. Considering these variables, many manufacturers do not recommend replacing the cooler if the leak is small. Until the leak is significant, engine performance is not compromised. After isolating and pressurizing the cooler to manufacturers specifications, the rate of leakage is checked against acceptable limits. Manufacturers typically recommend pressurizing the cooler to 30-psi and measuring the rate of leakage by observing pressure drop. Less than a 5-psi pressure drop within 15 seconds is acceptable leak limit for a variety of manufacturers. The typical leak rate from an intercooler should not exceed 5-psi in 15 seconds after pressurizing to 30-psi. Less leakage is acceptable Leak testing intercoolers requires pressurizing the cooler to 30-psi using a pressure regulator. After reaching the prescribed pressure the air supply is shut-off and the leakage rate measured.

13 397 Internal Restrictions Charge air cooler cores can be restricted for a variety of causes. For example, a shredded air filter gasket, water or oil ingestion can internally reduce airflow inside a cooler core. Oil can accumulate because crankcase emissions which can pass through the cooler condensing oil. Turbochargers wisp a small quantity of oil which can easily accumulate inside a core. Any dirt getting through an air filter can stick to the oily core tubes and further restrict air flow. A failed turbo may leave a large quantity of oil and even debris inside to cooler. It is important to have a cooler thoroughly flushed after a turbo failure to prevent ingestion of debris into an engine. Engines have been known to run-away to destruction on the lubrication oil loaded inside a cooler core. Cooler cores should be back-flushed with solvent and compressed air after a turbocharger failure of if a core has become internally restricted. Water can accumulate when driving through rain, snow and during conditions of high humidity such as fog. In some cases, the water can freeze causing ice blockage of the cooler core. Oil will accumulate from turbocharger oil wisp and closed crankcase ventilation systems. To evaluate whether excessive internal resistance to flow because of an internal restriction exists, a pressure drop test of the cooler is recommended. This is accomplished by measuring the inlet and outlet pressure of the cooler when the engine is under load. While monitoring the intake manifold pressure, a separate gauge can be installed on the cooler inlet or turbocharger outlet. Usually, no more than a 3-psi pressure drop is allowable. However, test results should be compared against manufacturer s specifications for acceptable limits. A pressure drop test measures internal restrictions of intercooler core. The pressures at the turbocharger outlet and intake manifold inlet are the test points. Testing is performed under full load at rated speed. Some pressure drop is expected but usually no more than 3- psi or 6-inches of mercury. The pressure drop across an intercooler can be measured under full load full speed.

14 398 External Restrictions Winter fronts, bug screens, blocked coolant radiators and A/C condensers can restrict external airflow over the charge air cooler. Sometimes a small leak in the cooler will allow oil to migrate out creating a adhesive surfaces for road dust to cling to surfaces behind a cooler. Even though a quick visual inspection of a cooler might show no signs of restriction, the area directly behind the cooler may be completely blocked. This type of restriction would also not allow sufficient airflow across the radiator causing engine overheating or high engine coolant temperatures. Engine emissions and cylinder heat loads would increase proportional to the degree of air restriction over the charge air cooler. To diagnose an external restriction, a temperature differential test can identify a loss of cooling across a cooler. The vehicle needs to travel at least 30mph. Normally there should not be more than a 20 or 30 F difference between ambient temperature and the intake manifold temperature. For comparative purposes, Cummins air temperature differential test limits the temperature difference between ambient air temperature and intake manifold temperature to no more than 50F or 28C with at least a 30 MPH (48 km/hr) air flow across the cooler. A temperature differential test across an intercooler measure cooler efficiency or whether the cooler is possibly externally restricted. The temperature difference between outside air and intake manifold should be no more than 50 degrees F. Techtip Testing For External Restrictions Intercooler temperature differential test can be easily accomplished on a road test with an electronic diagnostic tool. OEM software or hand held diagnostic code reader can usually monitor intake manifold temperature. This can be compared with the known outside temperature. Make sure to load the engine by operating it under full throttle up a grade or in a higher gear than the vehicle would normally operate. Hoses and Clamps ATAAC use unique connector hoses with their colour often differentiating the hot and cold side of the cooler. These hoses are made of chemically stable, durable silicone material. Clamps with smooth underside surfaces are used to prevent damage to the hose. Spring operated clamps are preferred since they can maintain a constant torque on the hose regardless of the dimensional variations produced by temperature extremes.

15 399 These T type clamps uses spring tension to apply an even smooth clamping force around the circumference of the intercooler hoses. Thermal expansion and contraction can loosen gear type clamps like those often found on the coolant hose. Under hood noises while under engine boost can be produced from leaking clamps and hoses. A sudden pressure drop in the intercooler caused by a blown clamp can cause oil to be drawn by the turbocharger compressor wheel. Update!! Ford 6.7 Waterpumps Left is primary right is secondary for liquid inter-stage cooling.

16 400 Two completely separate cooling systems which includes separate water pumps, radiators and coolant reservoirs caps and thermostats. Secondary system is for interstage air cooling. Number Component Number Component 7 Degas Bottle 1 Intercooler 4 EGR Cooler 6 Secondary Water Pump 5 Transmission Cooler 3 Fuel Cooler 2a Thermostat Housing 1 8 Secondary Radiator 2b Thermostat Housing 2