Throughout time, the internal combustion engines and fuel systems used in cars evolved. Carburettors were used for a long time until one day everything changed with the introduction of the more effective fuel injection. The first injection type was called throttle body injection (TBI) or single-point injection. It was characterized by a single-point injection layout in the throttle body. It was more efficient than a carburettor, but still much was left to be desired since the fuel may not get in equal amounts to all cylinders. We will not be discussing the throttle body injection system more in-depth as it was more of a prelude from the carburettors to the more sophisticated injection setups.
In this article, we will be looking at the evolution of the gasoline fuel injection, after the TBI, in terms of technology – from port (also called “indirect”) to direct to the dual-injection system. I will explain what each of these types of injection is characterized with and what are their benefits and drawbacks, making an overall comparison between them. Without further ado, let’s start with what port injection is.
Port Injection
So what is a port injection (PI)? In this injection setup, the fuel is sprayed onto the intake valves before they open. This allows the air, coming from the intake manifold, and fuel to mix before entering the combustion chamber, which provides ample time for a nice air-fuel mixture to happen, giving more efficient combustion.
To understand another benefit of the port injection, we have to first explore how the airflow in the engine works. To do that, you can refer to the rather rudimentary graphic above. To start with – every engine needs air to operate. This air is sucked from the atmosphere from a pipe and then sent to the air filter. After the filter does its job, the cleansed air proceeds to the mass airflow sensor (MAF). This component is meant to measure the amount of air coming into the engine. Then, it notifies the engine’s computer (or Engine control unit – ECU) so that the latter can decide how much fuel is needed for optimal combustion. After the air passes the MAF, it proceeds through tubing to the throttle body. Along the way, the air from the PCV recirculation system is added too. We will get to the PCV in a bit. The throttle body is the moving part which you control with the accelerator pedal. The more you press the pedal, the more open the throttle valve becomes. The more open the valve is – the more air is sucked into the system. After the throttle comes the intake manifold, which splits the air and routes it to each cylinder. Moving on, just before the air enters the combustion chamber, the fuel injector sprays the necessary amount of gasoline over the intake valves, mixing the petrol with the air. Now the intake valves open and the air-fuel mixture enters the chamber. The valve closes and the piston compresses the mix. The spark plug ignites it and the combustion pushes the piston back. During this phase though, some of the gases and unburned fuel from the combustion pass through the sides of the pistons, where the sealing piston rings are, and into the crankcase. These are called blow-by gases and have to be removed from the crankcase to avoid unnecessary compression, which will cause oil leaks. The positive crankcase ventilation system (PCV) is the device which recirculates these blow-by gases back into the air intake system. So far, so good. However, when these vapours mix with the fresh air from the air intake and hit the intake valves again, they form carbon deposits. If not cleaned, these deposits will hinder the movement of the intake valves, and the engine will have a harder time operating properly and may even stall. This issue is successfully tackled by the port injection layout because of the fact that the gasoline is sprayed onto the back of the intake valves. Modern gasoline has cleaning additives in it, so when the fuel is vaporized over the back of the intake valves, it cleans them from the gunk coming from the PCV.
This system is proven throughout the years, less complicated and sturdier. A drawback to the PI is that it uses a bit more fuel since it is not that precise, as the direct injection, as we will now see.
Direct Injection
What is a direct injection (DI)? In such an engine, the fuel is directly sprayed into the combustion chamber, where it mixes with the air. This provides less time for the air-fuel fusion to happen. The latter implies a higher pressure fuel pump in order for the gasoline to be delivered faster so the time for the mixture to be as long as possible.
The most important reason why the direct injection engine exists is for fuel economy and lesser emissions. The injectors, being inside the combustion chamber, means that the amount of sprayed fuel can be optimized. Also, you get an added benefit of cooling the combustion chamber. Apart from the around 10-15% better fuel efficiency, you also get more power. That’s great, however, studies have shown that a DI engine will exhaust more particulate matter (very tiny particles resulting from the combustion) in the air than a PI one. This is because the quality of the air-fuel mixture is reduced from the lesser amount of time for it to happen, and from here, the combustion produces more soot.
Looking at the scheme, the things are very similar to the port injection layout. However, not having the fuel injector spraying over the intake valve, there is nothing to stop the gunk accumulation that we already discussed.
There is one more drawback coming from the placement of the fuel injector inside the combustion chamber. When the engine stops working, and while the heat in the combustion chamber is still high from the last engine cycle, small amounts of not-entirely-burned fuel oxidize when mixing with fresh air from, let’s call it, the “last breath” of the engine before it being shut down. From here, more deposits are formed, this time over the injectors themselves. This can also happen in a PI engine, but much slower since the injectors in a PI system are not placed in the combustion chamber and hence, not subjected to the heat of the combustion.
The build-up of these carbon deposits is the fundamental downside of these types of engines. If not cleaned, the gunk causes rough idling, misfires and loss of power due to the obstruction of the air entering the engine. Apart from the above, the sludge can also cause loss of fuel efficiency and more emissions, which for me practically defeats the initial purpose of the existence of the direct injection petrol engine.
Usually, a DI engine’s intake valves need to be manually cleaned with a special cleaner every 30-60 thousand miles but sometimes sooner to prevent damage to the motor. Another thing you can do is to get your DI car a good catch can. This device is meant to seize the gunk sent back to the air intake system from the PCV valve and is placed between the PCV valve and the air intake. It will not catch all of the vapours, and it is still strongly recommended to clean your intake valves, but if it is a good one, it should stop at least 90%. In regards to the injectors, you should use injector cleaners from time to time to help keep them clean from the carbon deposits. The last thing, which is strongly recommended in a PI and a total must in a DI, is to get very high-quality oil on every change.
Another drawback to the DI engines is that, all things being equal, you can expect higher maintenance costs for them because of their more expensive and precise components like the high-pressure fuel pump. Other things to consider are that a DI engine is noisier than a PI setup and that the DI engines are prone to troubled cold starts. Also, port injection engines are better suited for high-speed operation and are easier to modify. Lastly, it’s fair to say that, again, other things being equal, we can expect that a PI engine will simply last longer than its direct-injection counterpart due to the lack of respiration problems from clogged intake valves of the indirect injection motors.
All of the above and the consistent reluctance of the car manufacturers to discuss the carbon deposits issues of their DI engines lead me to think that the gasoline DI system was established just to meet the regulatory requirements (which they do in a factory environment when the engine is new) so the car can be sold. If you think about it, everything looks great on paper. The new car’s brochure will brag about fewer emissions, better fuel economy and more power. It can be argued that this is understandable because, like any other company, the car manufacturers have to produce in order to survive but is there another feasible and more honest way? Continue reading, and at the end, we will discuss a not very familiar for most people alternative which may be possible for your area.
Dual Injection
What is a dual injection? In this system, the injection is double. The engine has port fuel injectors and direct injectors in all cylinders, getting the benefits of both setups for the cost of added complexity.
Apart from having the port injectors cleaning the gunk accumulated over the intake valves, there are other reasons for its existence. When setting these engines up, the engineers can utilize the different injectors in different circumstances. For instance, when the engine cold starts, it may use only the PI because, with this system, the engine starts better and has fewer emissions than a DI one. Another example is when the engine runs on higher RPM, when supposedly you need maximum power, it may run only the DI because of its higher power output and more significant cooling capabilities. On the other hand, in lower loads, the engine can utilize only PI to benefit from its cleaning ability of the intake valves and its advantages in proper air-fuel mixing.
Some issues inherited from the DI still remain with the dual injection, though. There is still nothing cleaning the injectors in the combustion chamber from oxidized fuel left-overs after engine shut-down, and the cleaning of the intake valves may not be as efficient as in a regular PI motor since the port injectors don’t work all the time.
The Alternative
Can we save ourselves from the DI drawbacks while still getting the fuel economy? This depends primarily on whether you are willing to drive maybe an older PI car and if you can get alternative fuels in your area. The alternative combustible I will talk most about is LPG (liquid petroleum gas), but you can also check CNG (compressed natural gas). These fuels burn significantly cleaner (with less particulate matter) than gasoline and diesel, have lower carbon content and are much cheaper.
The LPG, also called autogas, is a by-product of petroleum refining. In the past, it was released in the atmosphere. After a while, though, it was realized that it can be used as a fuel and a very clean one too. How clean, you may ask. According to the “World Liquid Propane Gas Association”, the LPG produces 21% less CO2 emissions and 81% less particular number emissions than petrol. It also creates between 60% and 70% fewer smog-producing hydrocarbons and 96% fewer toxins and carcinogens, like benzene and toluene, according to the “Southwest Research Institute”. Not only this, but due to the fact that LPG burns much cleaner than diesel and petrol, the issue with the valve gunk accompanying the DI engines is avoided.
For our car needs, the autogas is stored in a separate fuel tank under pressure so that it takes less space. Unfortunately, the LPG cars’ poor reputation of being “bombs on wheels” normally rebuffs many people from utilizing it. However, you should know that modern autogas systems are heavily regulated and rigorously tested. Contemporary LPG systems have so many defences in an event of an accident that it is significantly more likely the cause of a potential explosion to be the gasoline in the car. Suppose the tank’s pressure is increased due to damage caused by a collision or higher temperature caused by fire. In that case, the tank will start to release the gas into the atmosphere in a controlled way to prevent an explosion. ADAC (Europe’s largest motoring association) is one of the institutions that performed thorough testing in the near past. They tested the endurance of an LPG installation in two situations. Firstly, in an event of a rear-end (where the tank will usually be located) collision with 60 km/h and a separate second test in case of a fire beneath the vehicle. Both trials concluded that at no time the occupants of the car were in danger from the LPG installation. You can see the video from the test on ADAC’s Youtube channel in German here. Another example is the test performed by the alternative energy organization “Gazeo”. They too did a rear-end collision test with 54 km/h, but this time both cars were equipped with LPG, so the car which hit the rear of the other had gas in the engine bay. This means that the experiment tested not only the front car’s tank rigidity but also the rear car’s engine installation safety. You can see the test again on Youtube here.
The drawbacks to the LPG usage in car engines come from its higher octane rating. Autogas has an octane rating of 106 and hence it has a slower burning rate than gasoline. The good side from this is that there is less chance of a knock (uneven fuel detonation in the combustion chamber, which may lead to catastrophic consequences for the engine). Contrary to this, however, and the conception that due to the cleaner burning, the LPG usage actually prolongs engine life, a study performed by the Indian “Maulana Azad National Institute of Technology” concluded that utilizing LPG can “cause some damages on engine structural elements”. Maybe most commonly, though, the intake valves are the ones that will suffer the most (again). A potential valve burning, as it is called, happens when you deprive the valves of the cooling effect of the gasoline being sprayed over them. Having LPG being sprayed as a vapour (like the more common installations do) does not cool the valves, and they simply get hotter and hotter until they start to melt. This may be averted in two ways. The first is installing a liquid LPG injection system rather than a vapour one. LPG as a liquid has even better cooling capabilities than gasoline. The second way to prevent valve burning is the installation of hardened intake valves for your car.
Another thing to keep in mind if you decide to go LPG is that this alternative fuel can reduce the engine efficiency in terms of power and fuel consumption. It should be pointed out though, that the power loss can be hard to detect in most normal cars and can be mostly tackled or mitigated by a good professional installation of the LPG system. The reduction of fuel economy is also something that should not bother since firstly, the cost of LPG can be more than 50-60% less than the one of gasoline in some countries and secondly, the increase in fuel consumption is about 10-15%. Basically, you still get a massive discount.
If you are already wondering about LPG-ing your car, you should know that a PI engine is much easier and cheaper for conversion than a DI one. To add another fuel source for a DI engine, you have two options – an expensive one and a more expensive one. The less expensive will add LPG injectors where gasoline port injectors would be located, and in this setup, the car will have to run at all times on both fuels (LPG and gasoline) at the same time. This may not be possible for all vehicles as it requires the special ECU made by the manufacturer of the LPG system to have been specifically developed for the brand and model of the car. The more expensive option integrates the LPG injection in the gasoline injectors of the engine. Prices differ from country to country, but on average, if you go with a four-cylinder PI engine, you can expect to pay between $500 for a low-grade system to $1500 for a high-grade one. The return on investment is easy to be calculated depending on the distance you cover with your car each year.
Knowing all of the above, you may now be wondering why isn’t LPG a more prominent part of our fuel usage. Maybe some of you reading this haven’t even heard of this alternative so far at all. Not only this, but in some developed countries like the UK, autogas supply was cut off by some big fuel companies like Shell. The stated reason – “low demand and prohibitive compliance costs”. As already mentioned, I can understand the low demand coming from a lack of public awareness. However, the prohibitive compliance costs is a big question. The reason for the lack of government support and raising awareness for LPG, and CNG in this respect, in most parts of the world, is unknown to me. Of course, we can speculate that maybe it is a matter of interests and lobbies but, to be honest, I don’t have a definitive answer on why is that. The increasing production of electric cars may be there, but we are still very far from relying primarily on electricity for our automobiles. Also, another known truth of the matter is that battery production produces enormous amounts of CO2 emissions.
The bottom line is this: We still have a way to go with internal combustion engines, so why not make the road to the future much cleaner and cheaper for us and our children by going LPG or CNG, rather than using factory-flawed gasoline fuel injection systems like the DI or dirty and expensive fuel like diesel? The change comes from us – the consumers, representing the demand in the economy of our countries.
I hope you have found this article useful. Stay tuned for more!