Two Guys Rally

Gas Saving Tip #4 [ July 24th, 2008 ] By:Charles Smith

While this is not so much a gas saving tip as it is a money saving tip, I think it addresses a common misconception about gasoline.

Stick to Low Octane Gas. If you don’t have a requirement for above 89, buy the lowest octane gas that will keep your car working. Octane has nothing to do with engine performance by itself. That is 93 octane will not make a car that only needs 87 octane run better.

Higher octane gas can put up with higher temperatures before it ignites, so high performance engines take advantage of this fact. They compress and heat up the gas and air more than a normal engine would in various ways. They could turbo/supercharge, increase compression in the cylinders, etc… All of those would require higher octane gas. If they used lower octane gas, the engine may start to knock. That means the gas is igniting before the spark and you can damage many parts of your engine when this happens.

So unless your engine requires it to prevent knocking (detonation), stick to the lower octanes.

Hunting for Performance Loss Causes [ May 20th, 2008 ] By:Mark Ozimek

Recently I have noticed a lack of power in the upper RPM range of my S70. Considering that it is now 10 years old and has just over 146k miles, I’m sure there are a whole bunch of things that are causing problems in one way or another, from dirty fuel injectors to leaks in the air hoses. Part of being a good driver or co-driver on a rally team is being able to quickly and accurately diagnose faults with your car that impact the performance. Also, being the engineering oriented car enthusiast that I am, I like knowing what is going on under the hood of my car.

So immediately, there is a list of probable causes to this performance reduction.

1. A hole or tear in an air hose.
2. Compressor bypass valve (CBV) failure.
3. Turbo control valve (TCV), AKA boost control solenoid (BCS) failure.
4. Significant restriction in the exhaust system.

A hole or tear in an air hose can cause a vacuum or boost leak, depending on where it is. This will cause the reading from the MAF sensor to be incorrect, and will throw off the base pressure upon which some of the turbocharger’s boost control devices operate on. A significant leak will cause the ECU to throw an error and turn on the Check Engine light, as well as significantly decrease fuel economy, neither of which have happened, so it’s actually rather likely that this isn’t the cause.

The compressor bypass valve (CBV) is a known weak spot, being integrated into the compressor housing, it has to endure a lot of heat and vibration. Tears in the valve diaphragm develop over time and allow air to circulate back to the compressor inlet when the engine is under load, which we definitely do not want.

Another part that is known to fail on this car is the turbo control valve (TCV), or boost control solenoid (BCS). Either name works, they’re the same thing. It operates based on a duty cycle from the ECU and the pressure difference before and after the compressor, and sends the ‘resultant’ pressure to the wastegate actuator to control the boost pressure. If this solenoid is stuck open, the wastegate will open before it should, and the turbo will never develop significant amounts of boost.

A restriction behind the turbocharger can also cause a significant performance decrease, for example, the catalytic converter failing and clogging up. Any backpressure on the turbine wheel increases how hard the turbine must work to generate a certain boost level by a lot, so this is also a sensitive spot. It is also the easiest to check for, and is what I am doing today.

As you can see, it is most likely related to the turbocharger. Unfortunately Volvo did not see the need to include a factory boost gauge in the instrumentation cluster, nor do I have a gauge on hand to test to make sure the pressure is at 10 psig like it should be.

So, with limited diagnostic resources at hand, there are really two choices I have; Check for leaks, or see if there is a restriction in the exhaust. I decided to check for the latter, since checking for vacuum leaks would entail replacing hoses to see if anything changes, and I do not have appropriate hose with me.

Starting off, there is a nice heat shield around the exhaust manifold and turbocharger to keep the engine bay temps down a bit, so I’ll remove that to get to the downpipe which I will unbolt to allow for gas to flow directly out of the turbine housing, bypassing any restriction. Be sure to do this work when the car is cold, as the exhaust system can get VERY hot.

Now with the heat shield out of the way, I have access to the bolts holding the downpipe on. A 13mm socket fit perfectly. After loosening the bolts, I pulled the downpipe off the turbo outlet flange:

After pulling it back enough to create a significant gap, I started the car, and went for a short test drive. I could tell that a significant amount of air was bypassing the exhaust system through that gap from the sound of the air blowing through, and the exhaust smell in the cabin. I ended up opening the windows to get some fresh air, since carbon monoxide poisoning sucks.

After doing some hard acceleration after the car had warmed up, I found that there was very little difference in the performance of the car. It had become significantly louder though, quite similar to the sound of a diesel truck, and surely in violation of noise ordinances. Fortunately, that rules out restriction in the exhaust causing the performance issues, since it would have been an expensive fix, working against the money Charles and I are trying to save up for the rally car. So I put everything back together. Always make sure to use proper torque for tightening things up in the exhaust to prevent leaks or cracking. Volvo specifies 30 Nm of torque for the downpipe bolts.

Before putting the heat shield back on, I decided to investigate two of my other leads: the CBV and the TCV. The CBV is totally caked in dirt and oil, which makes me highly suspicious of a leak there. The only way to find out if it has failed would be to replace it, which I plan on doing since it is an inexpensive part.

The TCV was harder to look at, since it was hiding underneath the air intake hose. It did appear normal aside from the electrical tape, but looks can be deceiving, so this is next in line if replacing the CBV diaphragm doesn’t do anything.

It looks like I need to brush up on my diagnostic skills a bit, since I was not able to find the fault in an afternoon’s work. When I find out what is wrong, I’ll post a guide on what happened and how to fix it, since boost related performance issues appear all the time on turbocharged cars as they age.

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.