Before we dive into the details, let’s first clarify a fundamental concept:

The output capacity of an engine depends on its air intake capacity.

Let’s repeat that:

The output capacity of an engine depends on its air intake capacity.

The output capacity of an engine depends on its air intake capacity.

Understanding this statement will help you grasp many concepts related to car engines. Car engines are internal combustion engines, where fuel burns inside the cylinders, creating high-temperature and high-pressure gases that push the pistons to do work. This reciprocating motion of the pistons is converted into rotational motion by the crankshaft, ultimately outputting mechanical energy.

A term we often use is “fueling.” This may create the illusion that, to increase engine power, we simply need to spray more fuel into the cylinders.

However, in reality, when we step on the gas pedal, we’re actually adjusting the throttle valve’s opening, allowing more air to enter the engine. Only after the engine has confirmed that more air has entered will it increase fuel injection, and then you’ll feel the power boost.

Throttle valve
Throttle valve

Understanding this point allows us to continue our discussion, as all subsequent content revolves around engine air intake.

Let’s talk about displacement.

displacement
Displacement

Engine displacement is a purely physical space concept. The displacement formula is:

Displacement = Cylinder Working Volume × Number of Cylinders

The cylinder working volume formula is:

Cylinder Working Volume = Engine Stroke × Cylinder Cross-sectional Area = Stroke × π × (Cylinder Diameter)² ÷ 4

The specific physical meaning is that when the piston moves from the top dead center to the bottom dead center, the physical space inside the engine cylinder is the displacement. Some answers suggest that displacement represents the “fluid volume inhaled or exhaled per stroke or per cycle.” This view is somewhat flawed, or at least incomplete in its definition. For gases, the concept of volume must coexist with the concepts of temperature and pressure. Displacement is merely a physical space definition and does not equal the actual amount of air inhaled or the volume of post-combustion gases exhaled. The simplest example: Both 2.0T and 2.0L engines have a displacement of 2.0L, but the power of a 2.0T engine is about 50% higher than that of a 2.0L engine. The fundamental difference does not come from the 2.0T engine injecting 50% more fuel than the 2.0L engine but rather from the 2.0T engine taking in 50% more air.

The formula for the volume of air inhaled per cylinder per cycle (under environmental pressure and temperature conditions) is:

Cylinder Air Intake Volume (Environmental Pressure/Temperature) = Cylinder Displacement × Filling Coefficient × Intake Manifold Coefficient

The filling coefficient is defined as: the ratio of the volume of air inhaled per cylinder per cycle, converted to the intake manifold state (pressure/temperature), to the physical displacement of a single cylinder.

The intake manifold coefficient is defined as: the ratio of the volume of a unit of air under environmental atmospheric pressure/temperature to the volume of that air under intake manifold state (temperature/pressure).

A simple illustration is as follows :

Volumetric efficiency and intake system efficiency
Volumetric efficiency and intake system efficiency

If we equate environmental atmospheric pressure/temperature with standard atmospheric pressure and temperature, then we can establish a standard correspondence between the volume and quantity of fresh air. This way, the impact of displacement on the engine’s ability to perform work, as well as the things we can do, become clear.

Recall the statement I made at the beginning: the output capacity of an engine depends on its air intake capacity. Engine displacement is the physical foundation of an engine’s ability to perform work. With the same technical configuration, the larger the displacement, the greater the engine’s power and torque output capacity.

However, displacement only determines volume, and as I mentioned earlier: for gases, the concept of volume must coexist with the concepts of temperature and pressure.

Take a look at this illustration:

Volumetric efficiency and intake system efficiency

The size of V2 is directly affected by Vs. However, the size of V0 depends not only on V2 or Vs but also on the differences between P2 and T2 and the environmental temperature and pressure.

In simpler terms, with the same displacement, the actual amount of air intake per cycle in the engine cylinder is related to the intake temperature (T2) and the intake pressure (P2). The lower the intake temperature and the higher the intake pressure, the more air intake, and the stronger the engine’s ability to perform work.

Let’s discuss intake pressure.

For naturally aspirated engines, all the power of the air intake comes from the vacuum created during the intake stroke. Therefore, it becomes essential to elegantly design the intake manifold, reduce air intake resistance, and utilize intake resonance while avoiding interference with intake efficiency. Generally, engines with more than 1.5L displacement use variable-length intake manifolds for optimization.

This is why naturally aspirated engines often have a complex and large intake manifold structure, while turbocharged engines are relatively simple. Turbocharged engines are more straightforward, as they rely on direct pressurization, reducing the need for extensive airflow research in the intake manifold.

Mechanical supercharger
Mechanical supercharger
Wastegate turbocharger
Wastegate turbocharger

Furthermore, Atkinson cycle and Miller cycle engines, which adjust the valve mechanism to close the intake valve early during the intake stroke or keep the exhaust valve open after the start of the compression stroke, can also reduce the actual intake volume. This leads to a situation where the expansion ratio is greater than the compression ratio and creates a discrepancy between the displacement and the actual intake volume.

Let’s discuss controlling the intake temperature (T2).

Simply put, the goal is to lower the intake temperature as much as possible to increase air density.

This is especially important for turbocharged engines. As the vehicle takes in air from the environment, the air is heated by the exhaust gas temperature during the turbocharging process. This results in a higher intake temperature, which in turn leads to insufficient air density and a lower actual intake volume for the same volume. To address this issue, cooling is needed, and this engine structure is called an intercooler.

The intercooler used in most engines is an air-to-air intercooler, which cools the intake air using air as the coolant. This component is typically part of the front cabin structure of the vehicle and looks something like this:

Air cooling is definitely not as good as water cooling. For high-performance engines, the proportion of water cooling has been increasing in recent years, which further improves torque response.

Let’s summarize:

The displacement of a single cylinder is the physical space between the piston up and down strokes in the cylinder of the engine. It is the basis for the intake volume of an engine. By boosting, reducing intake resistance, adjusting the intake and exhaust valve profiles themselves and phasing strategies, and changing the intake air temperature through intercoolers, the actual intake volume can be further changed. And the actual intake volume determines the performance of the engine. 

Let’s talk about horsepower, power and torque.

Horsepower is a unit of power.

The earliest definition of horsepower was: a horse with a traction force of 180 pounds can pull a waterwheel with a radius of 12 feet 144 revolutions in one hour, calculated as 33,000 feet • pounds/minute, he named it 1 horsepower, which is equivalent to 746W now.

horsepower
Horsepower

The definitions of power and torque:

Academically, it should be called the effective power of the engine, which is defined as: the effective work done by the engine in a unit of time.
The effective power of the engine is obtained through a dynamometer test under specific working conditions. The output torque Ttq and engine speed n of the engine. Power = Torque * Rotational speed
If the unit of effective power Pe is kW and the unit of torque Ttq is Nm, and the unit of speed n is rpm (revolutions/minute), then the formula is:

Torque is a kind of rotational moment, while power considers the total amount of work in a unit of time.
Many people are confused about power and torque, essentially because they do not understand whether to look at power or torque to see how a car’s performance is. 

Previously, an automotive engineer discussed this issue with many people specifically. He wanted to explain that the 0-100 acceleration of a vehicle is mainly determined by power rather than torque. The reason for this discussion is that German vehicles generally use turbocharging and other methods to improve low-speed torque response, but power is generally not high (and has decreased rapidly in recent years), while many large displacement naturally aspirated engines have higher power. Many automakers are also learning German methods. However, the engineer believes that power determines the performance of the vehicle, especially the 0-100 acceleration.

This issue looks simple, but due to the vehicle’s transmission gear shifting strategy and the fact that the engine needs to consider the combination of maximum power, maximum torque and torque curve to get the result, it is actually a difficult question to answer.

Based on our actual development experience, I give two simple and crude conclusions:

0-40 acceleration, torque priority; 40-100 acceleration, power priority; 0-100 acceleration, power priority.
Torque depends not only on maximum torque, but also on the torque curve with engine speed. Combining torque and peak torque with the width of the torque curve platform can link power and torque to vehicle performance.

The reason why the German system is so obsessed with low torque is brought about by Germany’s national conditions. Because my former company once had an engine that was originally planned for sale in Europe, my European colleagues gave me some European market needs at that time. Among them, one left a deep impression on me. About 10% of the European market share came from company vehicles. Company vehicle drivers were very concerned about the acceleration response after starting a pedal, which is the main role of low-speed torque. So I kept asking to improve the minimum speed of maximum torque and torque response.

In terms of power performance comparison, I want to talk about 3 points:

1. Is power or torque a priority for power performance?
2. What affects low torque response?
3. Is torque just looking at a maximum torque?

To face the problem directly, let’s start with the 3rd point: torque curve tuning:

Putting the torque data of these four engines together with Honda 1.5T, we can see more clearly:

Torque data for four engines
SAIC GM New 1.5T VS Chery 1.6T/Great Wall 1.5T/Mercedes-Benz M264 1.5T/Honda 1.5T 260 Turbo

Interpret what this table represents:

• The orange-yellow is Chery 1.6T. In terms of torque, the displacement advantage gave it some advantages in peak torque, but the peak torque appeared late, and the peak torque platform was 2000-4000rpm. The displacement advantage gives the total torque advantage, but also brings the turbocharged torque climbing problem, and the intake end is not optimized much. Overall, the performance is mediocre.  (Chery’s data comes from Chery’s 1.6T PPT chart. The original chart shows that the peak torque of 290Nm@2000rpm slightly drifts down to 285Nm@4000rpm. This is similar to Great Wall’s torque data drifting slightly up to 285Nm at 1750rpm, and the overall torque platform is around 280-283Nm.)

• The red is Mercedes-Benz 1.5T data. This torque curve looks very strange. Mainly, I did not actually find other power torque diagrams of the M264. This chart corresponds to the information in their official data this time that the peak torque only appears between 3000 and 4000, and this information is correct. Like Chery, although 280Nm of torque was squeezed out of 1.5T, it appeared too late and lasted too short.

• Great Wall 1.5T is another extreme. The torque increases quickly with the increase of speed at low speed, but it leaks before 3000 rpm. This performance in the vehicle means that the starting feeling is good, but frequent gear changes will occur during medium and high speed competitive driving. Without a stable torque platform, it means greater transmission adjustment requirements. This is related to Great Wall’s choice of turbochargers and vehicle tuning direction. It seems that gasoline engines have been tuned to have the taste of diesel engines.

The reason for adding Honda 1.5T high-power version with lower power and torque than GM’s new 1.5T is that GM’s new 1.5T engine tuning concept is quite different from Honda’s 1.5T high-power version. GM’s new 1.5T reaches 95% torque at 1500rpm, close to the peak torque of Honda’s 1.5T high-power version. In contrast, the ultra-wide peak torque platform corresponds to the abundant and sustained power of the vehicle in medium and high speed racing, avoiding impact gear shifting and power interruption during key acceleration!

More importantly, GM’s high-power 1.5T is developed with a 48V system. At present, SAIC GM has not officially announced the data of the motor, but referring to the level of similar BSG motors, the peak torque is expected to reach about 50Nm, as the low-speed torque compensation of the engine, quickly intervening.

Honda’s 260Turbo version has been well received in North America since its launch. At present, General Motors and Honda actually have a very good technical cooperation model in engine development and automotive power tuning. In September 2020, the two companies signed a non-binding strategic cooperation agreement to jointly develop gasoline-powered and all-electric vehicles.

In recent years, GM has vigorously promoted 9AT/10AT in medium and high torque, and CVT and some 6AT in medium and small torque, which means combining Japanese smooth power tuning with American high-performance tuning. SAIC GM’s 9AT performance is the best proof. GM’s latest generation of R&D systems adopts a more comprehensive ARM driving quality. It aims to combine the power output of the engine with the tuning of the transmission and jointly develop the overall driving quality of the vehicle. This is not a simple problem of wiping a peak point, but pays more attention to the actual driving quality. 

Next, let’s go back to the first point, is power or torque a priority for power performance? 

I will directly throw out the conclusion: The power performance of the whole vehicle is the result of the combined output of the engine and transmission. In general, the power performance of 0-40kph depends more on torque, the power performance of 40-100kph depends more on power, and naturally the influence of power is greater at higher vehicle speeds.

Then comes the second point: What affects the low-speed torque performance?

In order to achieve stronger power and torque output, a larger turbocharged intake volume is required. After the engine speed increases, the intake time per cylinder decreases rapidly. To maintain stable power and torque output, a stronger intake is required. So you must understand this principle: the actual power of the engine is not determined by the injector, but by the intake system including the turbocharger. If this problem is only considered from the turbocharger design, either you choose a larger turbocharger, but the low-speed response will be slower, or you choose a smaller turbocharger, but the problem of insufficient boost and premature torque leakage will be encountered after the speed increases. It is even more difficult to obtain a peak torque platform. Either the peak torque is made low directly, but you can see that if the torque platform is defined according to GM’s 1.5T 1750-5500rpm standard, the peak torque of these high-torque engines cannot be seen.

Summary

*The water-cooled intercooler intake system generally appears on German low-torque series. By reducing the intake air temperature, it achieves greater air density. Under the same turbocharger fan effect, fresh air can be pressed in faster and in greater amounts. *Water cooling can not only increase maximum power, but also greatly reduce the required volume of the intake system and improve the turbocharger response by over 17%.

*The 35Mpa high-efficiency combustion system is a magic weapon. *We talked about this in the second point. Simply put, this system and the control logic based on advanced injection algorithms can allow combustion control to avoid many emission and safety limitations and achieve faster combustion control response.

*The Dual Fast cam phaser is a product jointly developed by General Motors and suppliers through strategic cooperation. It is the world’s first to achieve a doubling of the opening and closing phase switching speed of the intake and exhaust valves. *The engine can enter the target state faster. 

*A low exhaust pressure, high-response exhaust system with optimized responsiveness, and this exhaust system contains a highly integrated GPF. When regulations and vehicle performance require GPF, this arrangement enables GPF to reach the appropriate temperature and work normally as soon as possible even under low load conditions, combining the 35Mpa system and intelligent combustion control. Good original exhaust brings the maximum risk reduction at the customer.

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