Two Guys Rally

Turbochargers! - Part 1 [ April 28th, 2008 ] By:Mark Ozimek

This is actually a very very complex topic with a lot of things to consider, so I will be splitting it up into several parts. The first one will cover the basics: What a turbocharger does, and how. If you’re already familiar with turbochargers… well, this might be a bit boring for you, but keep an eye out for the later articles where I get into more detail.

To put it very simply, a turbo increases the amount of air flowing into an engine to create more power. If you read the article on temperature and engine power, you’d see that compressing air (by increasing the ‘boost’) increases the density. This means a higher mass in the cylinders, which will be compensated for by more gasoline. The end result of all this is more power.

How does it do that? Well, pretty simply actually. A turbocharger is made up of two centrifugal turbine wheels. One is exposed to the exhaust gas, referred to as the turbine wheel, and the other is exposed to the intake air, and is referred to as the compressor. The two are connected by a shaft, and the whole assembly is free to rotate inside the housing. Typical rotational speeds range from 100,000 rpm to 200,000 rpm at normal boost and airflow rates at full throttle.

What happens is the exhaust gas flows from the turbine housing, into the turbine wheel, and then out the center of the turbine wheel towards the exhaust pipe. The force and velocity of the air from the engine creates enough power to spin the turbine. The compressor wheel works in the opposite fashion. Spinning at high speeds, it sucks in air through the center and expels it out the circumference of the wheel at a high pressure. The pressure of the air is commonly referred to as boost, and is measured in relative terms, meaning 0psig (g for gauge) is actually atmospheric pressure, usually 14.7psia (a for absolute). Then at say, 10psig of boost, the real pressure of the air in the intake stream is 24.7psia, assuming that the car is being driven at sea level. The difference between gauge and absolute pressure is trivial until we start trying to select a turbo for a specific application, so don’t worry about the absolute pressure until then.

For the visual learners among us, like myself, here’s a handy diagram from Garrett that makes the turbochargine process pretty simple:

The following steps are labeled on the diagram:

1. Air enters the compressor wheel
2. Compressed air leaves the compressor wheel
3. Heat is removed from compressed air (temperature rises when air is compressed)
4. Air enters cylinder
5. Hot exhaust gas leaves cylinder
6. Exhaust gas enters turbine wheel
7. Exhaust gas leaves turbine wheel after imparting some energy into making it spin

While turbochargers offer the ability to significantly increase power output with very little weight addition, there are a few downsides.

One often cited one is “turbo-lag”, which can refer to two different things. At lower engine speed, there is not enough airflow through the engine for the turbo to create a significant amount of boost. As a result, turbocharged engines typically do not have very much torque below 2500-3000rpm, unless the turbo is sized very small for the displacement. The other thing is actually lag, when the gas pedal is depressed, it takes some time for the turbine wheel to “spool up”, and spin faster to make more boost to meet the engine demands. Bigger turbochargers typically spool slower and need more engine speed to reach the target boost level, but is less restrictive on the exhaust and can ultimately yield more power in the high end of the rpm band. The opposite is true for smaller turbos.

Another is increased complexity. The ECU has to monitor the pressure going into the engine, and add more fuel, cut back on timing, and restrict the boost level as needed to prevent the engine from detonating or knocking with lots of boost. Left uncontrolled, the turbocharger will create upwards of 30-40psig, which most engines cannot handle.

So why use it? As I said before, it is an excellent way of increasing power output without adding very much weight. For rally racing, this is ideal, since we want to keep the cars as light as possible for good handling. Getting the power that the engines make now without a turbocharger will mean making engines at least 50% bigger, which is a lot more weight.

How Engines Respond to Air Temperature [ April 24th, 2008 ] By:Mark Ozimek

I’m sure that everyone has heard or experienced first hand how cars have more power when it is colder outside. This may strike some of you as being counter-intuitive, but from a technical standpoint, it makes a lot of sense. Let’s break it down to see what is happening when it gets cold out.

The engines in cars burn gasoline (or diesel) and oxygen to create energy. The heat created by this explosion causes the air inside the cylinder to expand, pressing down on the piston, which creates a torque in the crankshaft. There is a very specific ratio between the amount of gasoline and air that provides peak power. This ratio is called the Air/Fuel Ratio or AFR, for obvious reasons. Get too far away from this ratio, and the gasoline won’t even ignite! As a general rule of thumb, engines run with around 12 to 16 pounds of air per pound of gasoline. Below 14.7:1 is called “rich”, while above that is “lean”, while 14.7:1 is “stoichiometric” for normal gasoline. Typically cars run lean for better economy when subjected to low load conditions, and rich when the pedal is to the floor, to help prevent overheating of the engine. Getting into optimal AFRs will be an article for another day though.

Why does all that matter? Well, notice that it is pounds of air and fuel. Engines have a constant volumetric displacement, but it’s possible to vary the mass of air flowing into the engine by changing the density of the air flowing into the engine. In fact, this is exactly how a throttle valve works. When the throttle is partially open, the air flow is restricted such that the density of air in the cylinders is very low, so there is very little mass. The ECU is aware of how much air is in there and injects an amount of fuel to match to get close to the AFR that it wants. A turbocharger or supercharger takes this in the opposite direction and increases the density of air by compressing it.

All of this has a point, don’t worry! Many of you have surely taken some sort of chemistry class, either in high school or college. To make things simple, air can be modeled as an “ideal gas”, which means we can easily say how four very important things relate as you change one of them: Density, Volume, Temperature and Pressure. The density and temperature terms are the critical ones for explaining why engines have more power when it is cold out. As temperature decreases, the density increases. That means when it is cold out, the engine will be able to contain a higher mass of air than at a higher ambient temperature, assuming the same throttle position and engine speed. When there is more air, there is more force from it’s expansion when the gasoline burns, which translates to more torque and power.

We can take this a bit further though. Notice the pressure term. At sea level, the air pressure is much higher than it will be somewhere like Independence Pass, Colorado, at 12,095ft. At this point, the pressure is down to about 60% of the pressure at sea level. Pressure and density have a direct relationship, so at 60% of sea level pressure, the engine is flowing 40% less mass, which will then correspond to a loss of about 40% of the engine’s power that it had at sea level (This is a rough approximation and it will be more or less depending on load and engine design. Having a turbocharger is a good way to get around this loss in pressure, but a significant power reduction will still be there). Pretty substantial, isn’t it? Now you know why the WRC racers were complaining about how slow the cars felt in Mexico, as they were racing at around 6,000ft for much of the course.

Another thing to note: With lower air temperatures, while the engine power increases due to the increased density, the drag from air resistance increases as well. The body of the car has to push more dense air out of the way as it is moving forward. This will negate a lot of the benefits of the extra power at higher speeds, but drag is a very minor part of the forces a car must overcome when accelerating at low speed. A similar effect is going on at high altitudes.

Ditch Gasoline?! [ April 14th, 2008 ] By:Mark Ozimek

So Charles and I were talking about the engines in rally cars the other day, specifically how the horsepower in the WRC is limited to 300hp. If you have read my article on engine power and torque, you’ll realize that this means they have the option to make amazing amounts of torque at lower rpm while staying under that 300hp limit. It is possible to design a gasoline engine to stay close to 300hp for a good part of the rpm band. Just size the turbo right so that there is a lot of boost down low and use a control system to taper off the boost in higher rpm to not exceed the power limits, coupled with a well-configured camshaft profile and such.

This type of power output curve very closely resembles that of an electric motor. The most torque is seen at or very close to 0 rpm, with the power output being pretty constant through the rpm range. This torque makes an electric motor great for starting off the line, or coming out of slow corners with lots of acceleration. There is another aspect of an electric motor that makes it far superior for the responsiveness that rally racing requires: the power response is instantaneous. With an internal combustion engine (abbreviated ICE), the throttle valve opens when you push the pedal down. This allows more air to flow into the engine, the ECU will see this through various monitoring methods and inject for fuel to keep the mixture close to what is required for the conditions (usually around 12:1 to 14:1). Then the exhaust gas flows through the turbine wheel, makes the turbine and compressor spin faster, increasing the pressure in the intake, causing even more air to enter the engine, creating more power. As you can see, there are quite a few steps involved here. The throttle response of a typical ICE is considered to be pretty fast, which is why they are used in cars, but in comparison, the electric motor is much must faster.

Simply press the accelerator pedal (not a throttle anymore!), the power control circuitry will allow more current to flow through the motor, and the motor creates more torque. No waiting for air to accelerate into the engine, no waiting for the ECU to compensate for this extra air with more fuel, no waiting for the exhaust to flow past the turbine to spool it up. It’s nearly instantaneous in comparison.

There are quite a few other advantages to electric motors aside from this. The thermal efficiency is often >80%, while a typical high performance ICE will be lucky to get 15% efficiency out of the gasoline it burns. The overall package size and weight of the engine itself is greatly in favor of the electric motor. Same with reliability: a gasoline engine has a lot of moving parts from the crankshaft up to the valvetrain, while an electric motor just has the core that spins.

The two of us sat there and discussed this topic for quite a while, and realized how amazing it would be. There is just one significant technical hurdle left to overcome: the batteries. Getting the range and power output needed for a rally car will weigh a lot. However, it is definitely something to consider as a possibility for the future of rally racing. What do you think about it? What other unforeseen problems do you think there would be? We would really like to hear what you think.

Engine performance: Torque and Horsepower [ April 8th, 2008 ] By:Mark Ozimek

Time and time again, I see people all over arguing the endless debate in engine design: Torque vs Horsepower, and which one is better. Although I am aware that ending this debate is impossible, and trying to do so would be quite insane, I wish to share what the engineer in me thinks matters most.

A quick crash course in physics for those who are not familiar with the topic. There is a very simple relationship between acceleration, mass and Force: F=m*a. Bear with me here, we have a lot of fun material to cover.

For cars, the force that causes the car to accelerate and decelerate (braking!) is generated by the tires and the ground. There is a torque at the wheels that causes the wheels to rotate. The fricton between the tire and the ground converts this torque into a force. This force is equal to the torque applied to the wheels divided by the radius of the wheel, with this radius measured from the center to the tire’s contact surface with the ground. This is assuming that the tire is not slipping. If it is slipping, then the force is dependant upon a lot of other things, like the surface conditions, friction coefficient, how fast the surface of the tire is sliding across the ground, temperature, and a few others.

With our intentions of rally racing, we obviously want the most acceleration possible to get out of slow corners quickly, while still having a fairly fast top speed for the straighter sections of the course. We can rewrite the relationship given above to be a = F/m, which implies that either increasing the Force or decreasing the mass will improve the acceleration of the car. The mass would simply be the mass of the car, more commonly refered to as the weight. I won’t get into how this affects acceleration yet, since this is an article on the engine, not weight reduction.

Now since more force determines acceleration, and the force increases with more torque, you’re most likely thinking: “AHA! So torque really does detemine the acceleration of the vehicle”. You are correct to think this, but there is a catch: it’s the torque at the wheels that matters. Taking it a step further, it is the engine torque and overall gear ratio that determines the torque at the wheels. This may seem obvious, but this is why acceleration is greater in 1st gear than a higher gear, such as 2nd or 4th. The gear ratio is much higher in lower gears, causing the torque that the engine generates to be multiplied by a factor of 8-15 in first down to around 1.5-3 in fifth gear, depending on the gearbox setup.

The consequence of this is that the output rpm from the transmission is much lower in the lower gears, so it is difficult to reach high speeds in low gears unless you’re using an F1 engine that hits the rev limiter at 19,000rpm. So now we have three factors to consider, the torque the engine is creating, the gear that the car is in, and the engine speed, to determine the acceleration of the car and the speed that it is traveling at.

So where does horsepower come into play? It’s quite simple actually. Using the imperial system, horsepower, torque and engine rpm can be related very simply: HP = (torque * rpm) / 5252. That bottom number is just the combination of factors used in unit conversion, since HP is a measure of power (who would have thought?!) which is an amount of work done per unit time. As a visual representation of the relationship between these three things, consider two power sources that I will use in an example later: One that puts out a constant amount of power, and one that puts out a constant amount of torque.

Realistically, an engine’s power and torque curves will look more like this, for a well configured turbocharged engine:

Work and torque are the same thing, in a twisted sense. Work is measured by a force applied over a certain distance. Pounds is a force and a foot is distance. Say you pressed on a block with 200 lbs of force over a distance of one foot. You just did 200 ft-lb of work.

To convert it into power, the time it took to do this amount of work is needed. Let’s assume that you’re a strong guy, and managed to do 200 ft-lb of work in just a half second. This means that you generated 400 ft-lb/s of power. Way back in the day, it was decided that there are 550 ft-lb/s of power in one horsepower. This means that you just generated 0.72hp when you moved that object in a half second.

To clarify power further, consider two different power sources, a turbocharged gasoline engine and an electric motor. The gasoline engine puts out a constant amount of torque through a broad rpm range (not really, but go with it for simplification of the explanation), while an electric engine puts out a pretty constant amount of horsepower, but has a very high RPM limit. To achieve the same range in speed, the turbocharged gasoline engine needs a gearbox to vary the ratio between the engine speed and wheel speed. The electric motor does not need this gearbox, as it has lots of torque at 0 rpm and can spin much faster. The electric motor will have a pretty smooth acceleration curve. The most acceleration will be seen when starting from a stop, due to the high torque at low rpm. As speed increases, the accleration tapers off because the motor creates a constant power; at high rpm, the torque is very low. On the other hand, the gasoline engine will produce an acceleration curve that looks like a step function. It generates a consistent amount of torque through the usable rpm range. As a result the acceleration in each gear is roughly constant while speeding up. When the driver shifts to a higher gear as the speed increases, the amount of torque to the wheels drops, thus decreasing the acceleration of the car.

How does all this fancy unit conversion relate to the rally car? Well, while the torque gets multiplied and changed along with the rpm through the transmission’s gear ratios, the power stays the same, minus some losses through friction, regardless of the gear. This power that the engine puts out can be directly equated to the power put into accelerating the car, overcoming the various drag forces, moving the entire car up and down hills, and so forth. This is where I introduce another equation, one that relates Energy with mass and velocity: E = (1/2)*m*v²

To avoid getting into calculus and integrating power with respect to time, just keep in mind that energy can be thought of as the total amount of power that has been applied to the system, which in this case, is the car. The more power the engine generates, the faster the velocity changes. The change in velocity is measured as… dundunDUN! The acceleration! However, note that the velocity term is squared. As the speed increases, the engine needs to create more and more power to maintain the same accleration. This should sound pretty familiar to something we found when calculating the acceleration with the torque: higher gears let you go faster, but decrease the amount of acceleration. There is no getting around this. So now we have two relatively simple ways of finding the acceleration of a car at any given moment:

1. The torque at the wheels, found by multiplying the engine torque and gear ratio.
2. The power output of the engine and the velocity of the car.

So in the end, what is it that really does matter? To such a difficult question, I find it neccessary to give a cryptic answer: it depends on what you’re trying to do! In a perfect world, engines would have infinite amounts of torque and power, and the acceleration would be limited by the friction coefficient of the tires. Unfortunately this isn’t the case, so we must settle on a compromise between power and torque. With common engine technology, the camshaft profile and timing has the largest easily changed effect on where the engine’s peak torque is in the rpm band. Due to this limitation, engines usually focus on low end, midrange or high end torque. The low end stuff is great for getting moving, especially if you’re moving a lot of weight. Good midrange torque makes for a very drivable car in almost all circumstances. High end torque translates to the most horsepower, which is good if you want to go really fast all the time, though it usually comes with the cost of reduced acceleration. One way to avoid this compromise is to use variable valve timing, but this is out of the scope of this article.

For high speed racing, like F1, having as much power as possible is what wins races. This can be seen by the design philosophy of the engines: Astronomical rev limits to get the most amount of power out of an engine that can develop limited amounts of torque. It’s not every day that you see normally aspirated 2.4L V8’s putting out 700-800hp. They can do that thanks to the rpm that the design allows. Remember that hp = torque * rpm / 5252. We can solve for torque in this case to find that at the rev limit, the F1 engines are making around 200 ft-lbs of torque. This is still very impressive for the displacement, but not nearly as high as the power output. Conversely, engines with a very low rpm limit, like diesel engines, must generate massive amounts of torque to make any reasonable amount of power.

For rally racing, having as much acceleration as possible available to you at any moment is imperative. As such, we want an engine that has a very broad torque and power curve with good responsiveness. Gobs of torque down low, without sacrificing the top end is ideal for maximum performance in the varied conditions that rally cars encounter. As such, compromises are usually made to focus on midrange torque, which will still offer decent low end and top end power. This is the design path we will follow when we start doing engine modifications to our car.