Regan Brown
Naturally aspirated engines and turbocharged engines have their own advantages and disadvantages, and it is important to understand the difference between the two. A naturally aspirated engine is an internal combustion engine in which the air intake depends on atmospheric pressure. In contrast, a turbocharged engine compresses the air flowing into the engine, allowing for more air and fuel in the cylinder, resulting in a bigger explosion and more power. This article will explore the differences between these two types of engines and provide insight into how to distinguish between their fuel injectors.
Characteristics of Natural Aspirated and Turbo Fuel Injectors
| Characteristics | Values |
|---|---|
| Air Intake | Natural Aspirated: Depends on atmospheric pressure |
| Turbo: Forced induction through a turbocharger | |
| Power Output | Natural Aspirated: Less |
| Turbo: Higher power output | |
| Weight | Natural Aspirated: Lighter |
| Turbo: Heavier | |
| Engine Response | Natural Aspirated: More direct throttle response |
| Turbo: Turbo lag | |
| Fuel Efficiency | Natural Aspirated: Less |
| Turbo: Better fuel efficiency | |
| Engine Sound | Natural Aspirated: Louder, instant response |
| Turbo: Quieter, dull drone | |
| Fuel Injector Sizing | Natural Aspirated: Low impedance injectors require a peak-and-hold injector driver |
| Turbo: High impedance injectors are more common |
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What You'll Learn
Injector sizing and selection
Fuel injectors are responsible for controlling the amount of fuel and, in some cases, air entering the engine. In naturally aspirated engines, the air intake depends solely on atmospheric pressure, and fuel injection is used to control the fuel amount, while air is typically not modulated. On the other hand, turbocharged engines use forced induction to increase the mass of intake air beyond what atmospheric pressure can achieve. This distinction influences the injector sizing and selection process.
When selecting a fuel injector, it is essential to consider the engine's power output, whether it is naturally aspirated or turbocharged, and the type of fuel used, such as gasoline, alcohol, or ethanol. FuelTech, for example, offers a Fuel Injector Calculator that assists in selecting the proper injector size. This calculator allows users to input the estimated horsepower, the number of fuel injectors, and the type of induction (naturally aspirated or turbocharged).
Additionally, injector sizing involves considering the injector's maximum capacity and flow rate. The injector must be able to supply enough fuel to meet the engine's demands, especially in high-performance applications. Factors such as fuel pressure and the injector's physical dimensions also play a role in sizing. The injector's capacity and flow rate can be converted between different units, such as cc/min to lbs/hr, to facilitate calculations and comparisons.
Moreover, the selection process should take into account the specific application and operating conditions of the engine. For example, racing applications may require constant flow MFI systems, while highway applications have different considerations, such as part-throttle fuel control limitations. The injector's compatibility with the engine's fuel control system, whether it is mechanical or electronic, is another crucial factor in the selection process.
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Mechanical vs electronic fuel injection
Mechanical fuel injection (MFI) and electronic fuel injection (EFI) systems differ in several ways. Mechanical FI systems use a rotary or inline pump to deliver individual fuel bursts to each cylinder. This is typically controlled through a mechanical linkage to the timing belt or gears, which open the injectors and control fuel pressure. Mechanical FI often requires tuning to achieve the optimal balance, which can be labour-intensive. However, once optimised, it performs well, especially in cruise conditions.
On the other hand, electronic control systems feature an electronic control unit (ECU) that monitors the engine's speed and load using sensors. The ECU adjusts the fuel injection rate to deliver the required power. This allows for very precise fuel injection amounts and multiple injections into the cylinders during the compression cycle, resulting in efficient control of engine speed and power output. Electronic systems offer stable performance across different power levels, from 100% power to idle, and make it easier to trim individual cylinders.
The choice between MFI and EFI depends on the specific application. Mechanical FI may be sufficient for constant speed or electric power generation applications, light loaders, and static equipment with relatively constant loads. It is commonly found in older OEM gasoline engines, such as Porsche and Mercedes, and in racing applications with unique air and fuel control requirements.
In contrast, electronic control systems are often required to meet emissions standards as they enable more accurate control of fuel combustion, reducing NOx and particulate emissions. They are generally preferred for heavy-load machines and provide better engine-machine integration, making them advantageous for excavators, loaders, and forklift trucks, where they improve performance and comfort for the operator.
While there may not be significant power differences between the two systems, electronic fuel injection offers more stable and consistent performance, ease of trimming, and improved emissions control compared to mechanical fuel injection.
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Forced induction systems
There are several types of forced induction systems, each with its own unique characteristics and advantages. Turbochargers, superchargers, and nitrous oxide (NOS) systems are the most common types of forced induction systems. Turbochargers use exhaust gases generated by the engine to drive a turbine, which then compresses the incoming air. This compressed air is then forced into the engine's combustion chamber, increasing power output. Superchargers, on the other hand, are driven by the engine's crankshaft, using a belt to spin an air compressor. This compressed air is then supplied directly to the engine, instantly boosting performance. NOS systems introduce a controlled amount of nitrous oxide into the engine's intake, significantly increasing power by enriching the air-fuel mixture and promoting more robust combustion.
When deciding whether to switch to a forced induction system, it's important to consider the potential risks and benefits. While forced induction can improve engine power, it also carries the risk of producing too much power, which can lead to engine failure if the engine cannot handle the boost. Additionally, considerations such as cooling the combustion chamber, preventing engine knock, and limiting NOx exhaust emissions may impact the fuel efficiency of forced induction engines, particularly in high-performance engines.
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Variable-valve control
Variable valve timing (VVT) is a method of altering the timing of a valve lift event in an internal combustion engine, which is often used to improve performance, fuel economy, and emissions. It is particularly useful for improving performance in naturally aspirated engines at partial loads and lower RPMs, where their optimised design for natural air intake can result in better fuel efficiency compared to turbocharged engines.
VVT systems allow for better control over the air intake, optimising performance across different RPM ranges. This is achieved by altering the timing of the valve opening and closing, relative to the position of the crankshaft. The camshaft, driven by the crankshaft through timing belts, gears, or chains, opens (lifts) the valves for a certain amount of time during each intake and exhaust cycle.
Early variable valve timing systems used stepped adjustments, with one timing for below 3500 RPM and another for above. More advanced systems offer continuous adjustment of the valve timing, allowing for infinite adjustments to be made.
There are several methods to achieve variable valve timing, including mechanical devices, electro-hydraulic systems, and camless systems. Early intake valve closing (EIVC) is one method to reduce pumping losses at low engine speed, high vacuum conditions, by closing the intake valve earlier than normal. This can reduce pumping losses by 40% and increase fuel economy by 7%, although it may also increase hydrocarbon emissions due to the lower temperature of the combustion chamber.
Another method is early/late exhaust valve closing, where the timing of the exhaust valve is manipulated to control the amount of exhaust gas left in the cylinder. By holding the exhaust valve open slightly longer, the cylinder can be emptied more and be ready for a larger air/fuel charge on the intake stroke. Conversely, closing the valve slightly early will increase fuel efficiency as more exhaust gas remains in the cylinder.
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Fuel flow and horsepower
The air-fuel ratio is the ratio of parts fuel to air flow entering the engine. If the airflow entering the engine is doubled, the fuel delivery must also double to maintain the same AFR level. This is where the term BSFC or brake-specific fuel consumption comes in. BSFC is the key to understanding an engine's efficiency in terms of fuel consumption with respect to horsepower. The lower the BSFC number, the greater the engine's efficiency.
For a 500-horsepower engine, the simplified calculation would require a 32-lb/hr injector. However, this calculation does not account for additional fuel flow or other potential errors. To ensure the injector is not too small, it is recommended to use a 70% duty cycle, which is the percentage of operation. This would mean sizing the injector for a 500-horsepower engine with a maximum 70% duty cycle, resulting in a 42-lb/hr injector.
For engines with an aftermarket turbo or supercharger, stock fuel injectors will not be able to deliver enough fuel to the engine and will decrease performance. It is important to upgrade the fuel system with performance injectors and a performance fuel pump to ensure optimal fuel flow and horsepower.
To determine the exact fuel injector size needed for a specific engine, it is recommended to use a fuel injector calculator, which takes into account the number of fuel injectors, estimated horsepower, and type of induction.
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Frequently asked questions
A naturally aspirated engine, also known as a normally aspirated engine, is an internal combustion engine in which air intake depends solely on atmospheric pressure and does not have forced induction through a turbocharger or a supercharger.
A turbocharged engine increases the power output of the engine without significantly increasing the engine’s weight. A turbocharger compresses the air flowing into the engine, allowing the engine to squeeze more air into the cylinder, and more air means room for more fuel. More air and more fuel in a cylinder means a bigger bang, creating more power from each explosion compared to a naturally aspirated engine.
It is important to know about fuel injector sizing and selection. For street applications, high impedance injectors are the more common selection. These injectors are driven by a saturated injector driver and have a resistance value of around 12 ohms. Low impedance injectors require a peak-and-hold injector driver, which is more expensive and typically used in aftermarket ECUs.
