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Carbon buildup in GDI engines

Monday, April 1, 2019 - 07:00
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The carbon accumulations within the intake port area will create turbulent airflow. Additionally, if the carbon deposits are not deposited uniformly, they can create additional turbulent airflow. It is important to understand that these carbon deposits do not need to be heavy in order to create many of these problems. On the GDI engine small carbon accumulations in the intake port area can cause drivability problems. Every racer that has ported heads on a flow bench will attest to the fact that very small changes made within the intake runner and intake port area will create flow differences; both good and bad. These uneven intake carbon accumulations rob power, torque and fuel economy.

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The turbulent air caused by carbon deposits is especially harmful in the GDI engine. This can best be understood by analyzing both the port fuel injection and GDI methods. When the port fuel injection method is utilized the fuel is injected directly at the back of the closed intake valve. The intake valve being the hottest part of the intake port, at 400°F to 800°F, will help vaporize some of the fuel so it can burn during the combustion process. Once the fuel is injected the intake valve opens, allowing the Air-Fuel mixture to be mixed by the swirling air movement past the valve. Additionally, the piston’s upward movement during the compression stroke forces this mixture together further mixing the air-fuel charge.   

What this accomplishes is a very well mixed air-fuel charge that is very close to a truly homogenous mixture, which means that the charge mixture (air and fuel) has a uniform composition throughout the cylinder. When the spark occurs, it takes the fuel beyond its auto-ignition point and the flame front propagates across the combustion chamber. If the air-fuel charge is unevenly mixed, the propagation of the flame front will be impeded.  This will cause incomplete combustion of the charge. If the air-fuel charge is homogenous this flame front will propagate through the combustion chamber allowing complete combustion to occur.  

Figure 4

In the GDI engine the fuel is directly injected into the cylinder. With this type of fuel injection there is no ability to premix the Air-Fuel charge prior to the intake valve opening. Additionally, the swirl or tumble effect as the intake valve opens cannot be utilized. Therefore, the airflow into the cylinder is critical. This airflow must enter the cylinder and swirl correctly in order to catch the aerosolized fuel and completely mix these two components together, as illustrated in Figure 4. Time is another constraint in the mixing of these two components together. There is very little time for the air charge to mix with the fuel delivery, so the conditions must be correct in order to get this event to occur properly.

If the GDI engine’s intake port area, and/or intake valve becomes carbonized with deposits to the point where it effects this incoming airflow, then the proper mixing of the air and fuel cannot take place. If this air charge is not properly formed the fuel mixing event will not create a good homogenous mixture which will lead to incomplete combustion.

Since the internal combustion engine is a heat engine, the fundamental operation of the device is the production and use of heat that can then be converted to mechanical energy. In these engines everything that is done prior to the combustion of the fuel type is to set up the air-fuel in the cylinder so the charge can be ignited, burned, and combusted. In the spark ignition gasoline engine, a well-mixed air-fuel blend will have a greater chemical conversion rate during the combustion process. If this mixture is not a homogenous charge the maximum chemical potential will not be converted into thermal energy and hence mechanical energy.  

Dealing with carbon build up

Therefore, it is imperative to keep carbon accumulations to a minimum in these GDI engines. But how can we accomplish this? Obviously, disassembly of the engine and hand cleaning is one possibility. In order to accomplish this the intake manifold will need to be remove from the head. Now that access to the intake port area is provided, rotate the engine until both valves for the port to be cleaned are closed. Now, using a plastic scraper carefully hand scrape the large carbon deposits from the port area. Do not put force behind the scraper. You are just removing the main body of carbon so the media blaster can be more effective. Once you are done use an air nozzle to blow out any remaining carbon from the port. Next, use a walnut shell blaster to clean the remaining carbon from the port area. Clean each cylinder’s intake port area while the intake is off. Additionally, while the intake manifold is off don’t forget to clean the intake runners. GDI engines can have large carbon accumulations within the manifold.

New media and or walnut shell blasters are available and provide good results. However, these solutions are time intensive and expensive. Due to these limitations this can only been done when the engine has large carbon accumulations that create severe drivability issues.

Another less labor intensive and less expensive option is to chemically clean these engines. It is always recommended to borescope the induction system before and after a cleaning. Never assume that because the engine has been cleaned that the carbon has been removed. This may allow you to think the carbon is not creating the drivability problem when in fact the carbon has not been change by the cleaning. Over the years chemical cleaning his has proven to not be a very effective method. Anyone that has checked the carbon deposits with a borescope before and after cleaning knows how ineffective the industry chemicals really are.

However, there have been recent developments in the chemicals and delivery systems that now provide excellent results on GDI engines. In order to remove these unwanted carbon deposits, we need to understand that this is a two-part problem. The first problem is the effectiveness of the chemical itself. The second problem is getting the chemical to the carbon deposits. Both the chemical and application method will need to be designed to work together in order to optimize results.

Revisiting chemical cleaning options

In order to understand how these carbon deposit can be chemically removed we must first understand the carbon itself. The carbon structures that are produced in the GDI are very different from engine to engine. The carbon accumulation within an engine will vary depending on many different variables such as; the type of hydrocarbons the fuel is made of, the detergents added to the fuel base, the type of hydrocarbons the motor oil is made of, the anti-friction additives added into the oil, the operating temperature of the engine, the pressure the carbon is produced under, the load on the engine, the engine drive time, the engine drive cycle, and the engine design. Each of these variables will affect the type of carbon that will be produced and amount of carbon accumulation within the engine.

Perhaps the largest contributor to these GDI carbon accumulations is the engine lubricant. In the GDI engine there are different anti-friction additives put into the oil base. The purpose of these additives is to help the oil during the extreme loads put on the cam lobe that drives the high-pressure fuel pump. The engine oil passes into the induction system through the Positive Crankcase Ventilation (PCV) system, along with the anti-friction additives that have been put in the oil base. These oil and anti-friction additives will change the carbon’s structure, thus different oils and additive packages will make different carbon types. 

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