In this edition, it’s all about AIRFLOW!

Welcome to Part 2 of the Teaching Engine Thermodynamics Series. The first step of the Otto Cycle is the Intake Stroke, occurring between points 1 and 2 on the Otto diagram. The piston begins its journey at Top Dead Center (TDC), or all the way at the top of its stroke. The crankshaft rotates and pulls the piston down as the intake valve opens, which pulls fresh air into the engine. As the piston moves downward, the volume of the cylinder increases, which is why we move to the right on the Otto Diagram. And since the intake valve is open, the idealized cycle assumes we are open to the atmosphere, and therefore this process is at constant atmospheric pressure, which is why we don’t move up or down on the Otto diagram.

The idealized Otto cycle assumes the intake step is isochoric, however in actuality it is not.  Friction losses from the flowing air and reduced cross sectional area of the flow path (the area around the valves) causes the pressure to drop as you move towards the cylinder. This friction is what is responsible for creating some engine vacuum on a gasoline engine, even at wide open throttle (WOT). If you can reduce the friction losses by smoothing out the flow path of the air, you can increase your engine’s air charge.

Since a gasoline engine is a combustion process (an exothermic chemical reaction with oxygen), the amount of air you can force into your engine directly controls how much fuel you can inject. Your engine’s computer controls the fuel flow to your engine based on the air flow, not the other way around. This important aspect of your engine is called the Air to Fuel Ratio (AFR).

The stoichiometric AFR required to burn gasoline is about 14.7:1. This means, for every 14.7 parts of air you can cram into your engine, you can inject 1 part of fuel to consume all of the oxygen.  In other words, if you get 14.7 grams of air, you can inject 1 gram of fuel. But why? Because Chemistry.

We will cover this in more detail during the Combustion Article later in this series. For now, more air = more fuel = more power.

Now we know that if the air charge drops, the amount of fuel we can burn also drops. Look what happens to our Otto Cycle:

 

Suddenly our combustion process (Steps 3 to 4) is shorter. Since our air flow was restricted, the computer also cut back on the fuel flow, and we created less heat and pressure in our combustion process. Now, instead of being able to do the work from steps 4 to 5, we can only do the reduced amount of work from our shorter combustion process to step 5 (shown in red above). Also, you can see the pressure drop during the intake stroke as the intake valve achieves maximum lift. All-in-all, Now we have less horsepower!

And you can see how our ideal engine starts to look more like a real-world, non-ideal engine shown below.

Engine Design

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So what does my engine do to compensate for these inefficiencies? Why do car engines now have more horsepower and fuel efficiency than older engines? Engineers have added features that allow for dramatically improved airflow.

Camshaft Profile

One important design aspect engineers consider is the profile of the camshaft. The Intake (and exhaust) valves on your engine are opened with the camshaft. Each camshaft “Lobe” has a certain amount of lift, and a certain amount of time lift occurs, called duration. More lift, and more duration = more time the intake valve is open = more airflow. A very aggressive cam with a-lot of valve lift or overlap also tends to cause a rough idle, which is something most average drivers don’t want.

The problem is that if you leave the intake valve open to long, it will still be open when the compression stroke begins, which would push your fresh air back out again. If you open the intake valve too early, it could open during the exhaust stroke, and you could push exhaust gas into your intake manifold. This problem is called “valve overlap”, and forces engineers to compromise between performance, efficiency and idle quality. Check out the Comp Cams diagram of what overlap looks like.

An excellent innovation that helped solve this problem is “Variable Valve Timing”. Variable valve timing can adjust when a valve opens, and on some systems can even adjust how long a valve stays open. This allows one camshaft to match a variety of engine conditions, like throttle position and RPM.  A variety of VVT technologies exist, and some manufacturers like Yamaha and Honda’s “VTEC” actually use a second camshaft and allows the engine to switch camshafts all together! This allows the engine to use a camshaft with good idle and economy characteristics at low RPM, and an aggressive camshaft with maximum airflow and power at high RPM!

Here are some examples of VVT technology in use today:

• VANOS: Variable Onckenwellen Steuerung (German-designed system used by BMW, Ford, Ferrari and Lamborghini)
• VTEC:Variable Valve Timing and Lift Electronic Control (Honda)
• MIVEC:Mitsubishi Innovative Valve timing and lift Electronic Control (used by Mitsubishi)
• VVT-i or VVTL-i:Variable Valve Timing and Lift with Intelligence (Toyota)
• VVL:Variable Valve Lift (used by Nissan)

Turbochargers

Another technology used to improve airflow is Turbochargers. While turbocharger technology is actually pretty old, it has recently become much more widespread because it improves engine efficiency AND power. Turbochargers use exhaust gas pressure to drive a fan blade which increases intake pressure. By INCREASING intake pressure, the opposite effect of friction losses is achieved, and the total amount of fuel and combustion pressure can be increased. Moreover, the exhaust pressure comes from steps 5-6 (heat rejection) in the Otto cycle, which is normally wasted to atmosphere. By capturing energy from the heat rejection step and re-applying it to the intake and compression steps, the total thermodynamic efficiency of the engine increases!

The downside is that this benefit comes at the price of increased manufacturing cost. Increased combustion without an increased engine size increases stress and heat on engine internals, so higher quality pistons, connecting rods and valves must be used. The turbocharger system is also expensive, since it operates at thousands of RPM.  Another downside is that turbo’s only work at high engine RPM, since you need alot of exhaust gas to provide the boost to the intake, so at low RPM you don’t get much benefit with a turbo. This phenomena is often called “Turbo Lag”. This means if you want low RPM power or a flat power band, a turbo won’t help you much.

Superchargers also increase the air charge to an engine, but they do not increase the efficiency of an engine. Unlike a turbo, superchargers are driven by the serpentine belt, which means they actually require engine horsepower to operate. The benefit is that they don’t require exhaust gas velocity to operate, so they can boost power at lower RPM compared to a turbo.

Computer control, higher quality metals and more precise manufacturing techniques and the desire to reduce weight and increase fuel economy have all allowed turbochargers to be much more widespread in modern engines.

The Problem

Over time, engine performance can be severely impacted by poor lifter performance, intake fouling, filter plugging, faulty sensors and more. Maintenance is the key to keeping your thermodynamic efficiency up.

What Can I Do?

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There are a few things the every day owner can do to increase airflow, and therefore power and efficiency.

Air Filter and Air Intake

Replace you air filter on schedule! This seems obvious, and yet is often overlooked, or not well understood. Air filter performance can be measured in 3 categories, including:

• Pressure drop (flow capactiy)
• Dirt holding capacity (how much dirt it can hold before pressure drop increases)
• Filtration efficiency (particle size removed)

Over time, as the filter becomes dirty, the pressure drop across the filter rises. If the pressure drop rises, air flow into the engine becomes restricted and your air charge drops. When that happens depends on which type and size filter you have. Generally, OEM type paper filters should be changed every 20-30k miles, or more frequently in very dusty conditions. The below picture is an example of pressure drop for a dirty (Saturated) element vs a clean one.

I’m sure you have heard about “Cold air intakes” and KNN type filters. Be aware that most of those products are snake oil, and often have a negative impact to performance rather than a positive one. Let’s talk about cold air intakes first.

Any modern car already has a “cold air intake”, because it pulls air from outside of the engine compartment where the air is considered “cold”. Enough said. It really is that simple. Don’t waste your money. If you want a BIGGER intake, that is a different conversation, but 99% of the time your air flow’s bottle neck is your intake valves and cylinder heads, not your air filter. Don’t you think the manufacturer would have made the intake 2% larger at the factory if it really gave the engine 10 more horsepower? This is a very competitive field after-all.

KNN, Cold Blue and other Cotton type air filters tend to have poor dirt holding capacity and often poor efficiency. This means that they require frequent cleaning or replacement, and if you don’t, they will actually perform worse than a factory paper filter. Oiled filters are also more difficult and expensive to maintain, requiring filter media oil and cleaning detergent every 10k miles.

Oiled filter is on the left, and a standard OEM paper filter is on the right. The paper filter is the obvious winner, offering more than twice the filter surface area.

What about a cone filter? The short answer is no. The cone shape can offer more filtration surface area, but they usually don’t. You’ll need to calculate the surface area to find out if it’s really worth it. Worse though, is that most aftermarket cone filter installations actually install inside the engine compartment, which increases the temperature of the air in the intake. PV=nRT, so increasing temperature decreases the ratio of mass to volume, otherwise known as density. Decreasing air density means less air to your engine, which means less fuel, which means less power.

This is actually less of a cold air intake than the factory setup was.

 

What Filter SHOULD I Get?

The surface area of an air filter is easily calculated. It is the pleat depth x # of pleats x 2. Since almost all air filter boxes are designed for a certain filter height, all you really need to do is count the number of pleats, and buy the filter with the most. This will give you the highest air flow and largest dirt holding capacity. Some cheap filters don’t have a glue strip down the center to keep the pleats separated. Over time the pleats collapse and cut off air flow, so we recommend looking for this feature. Not only will this perform the best, but it is also the cheapest option. *Tip: these concepts apply to your house’s air filter, vacuum cleaner’s air filter etc. as well.

Engine Internals

Now that we talked about air filters, let’s talk about how engine internals can affect your airflow.

Do you have a “direct Injection” engine? On these engines, the fuel injector is located in the piston instead of in the intake runner like normal. Unfortunately, the Crank Case Ventilation system (PCV valve, CCV valve etc) still gets drawn into the intake runner. This combination causes crank case oil and sludge to deposit on the back of the Intake valves, reducing airflow significantly. In effect, sludge and carbon fouling on the back of your intake valves has the same impact as a plugged or dirty air filter. On a traditional engine, the fuel injector located in the intake runner will continuously wash the back of the intake valve with gasoline, keeping it clean.

On a traditional engine, the fuel injector would spray into the intake port, not the combustion chamber.

For those with direct injection engines, you need to remove the intake manifold and clean the sludge and carbon off of the intake valves if you want to keep performance and efficiency up. Too bad they didn’t tell you this in the brochure! For those with a more traditional engine without “direct injection”, the gasoline does the work for you!

Now let’s talk about Valve lifters. Many vehicles have “solid lifters”. These lifters can not self adjust, and require valve lashing every 100k miles to make sure the clearance is correct. If you don’t, there will be play between the lifter and the intake valve which will reduce how far the intake valve can open. If the intake valve doesn’t open as much, you get less air into your engine, and therefore less power. The Duramax diesel and Ford Taurus SHO are examples of engines with solid lifters. If you have hydraulic lifters, they self adjust with oil pressure and no adjustment or valve lashing is needed!

PCV valves and EGR valves failing can also cause excessive exhaust gas and oil to enter your intake manifold, which reduces the amount of Oxygen available for combustion. If you have a problem here make sure to get it fixed.

What about tips for engine builders?

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Now we have some great options to play with.

First, it’s all about the cylinder heads and intake manifold. Get good flow test data for the cylinder heads you are interested in. Maximize the intake valve size. Remember that cylinder head flow at half-lift of the camshaft is very important, since you will pass this area twice with every stroke (once opening and once closing). Once you have good bench test data, you can find a camshaft that has the the amount of intake valve lift that performed the best.

For example:

Valve Lift
	Iron Eagle Intake	Platinum Intake	Iron Eagle Exhaust w/pipe	Platinum Exhaust w/pipe
0.100	65	64	59	57
0.200	129	128	112	111
0.300	182	180	139	140
0.400	226	221	166	172
0.500	258	253	178	189
0.600	255	273	186	198

 

Let’s take the Dart Iron Eagle Intake valve performance for example. Peak (maximum) airflow occurred at 0.5 inches of valve lift, and very little additional performance was achieve with an extra 0.1″ of lift. This means you would be sacrificing idle and low RPM quality with very little benefit on the top end if you went with 0.6″ of camshaft lift. In this case, if you were getting this cylinder head, look for a camshaft with intake valve lift just over 0.5″.

Aside from bottle necks, friction loss is the next culprit. Minimize airflow turbulence by port-matching the intake manifold and the cylinder head intake port. Try to buy an intake manifold port size that matches your cylinder head intake port size as closely as possible. Also polish the intake runners if you have the patience. Definitely make sure the gasket set you buy doesn’t cover the port anywhere, and bore it out if it does.

If you are building an old engine that had a flat tappet camshaft, converting to a roller camshaft will give you more air charge with the same amount of valve lift. This is because a roller camshaft has a roller bearing on the end of the lifter, which allows for a steeper camshaft profile. In effect, even if the valve lift is the same, the intake valve is open LONGER on the roller camshaft (The duration is larger). Neat!

Modern engines will also increase the number of valves per cylinder to increase the open area for airflow. It’s easy to see below that a modern multi-port head has more open area. The cylinder simple isn’t wide enough to make a single intake valve have more area than two smaller intake valves. This is why car companies will advertise things like “24 Valve” in the brochure and sometimes even add a badge on the side of the car. A 12V V6 will have a hard time making more power than a 24v V6. If the engine has variable valve timing, sometimes the manufacturer will add valves of different sizes so the engine can switch depending on the rpm and power requirements (smaller valves for lower RPM).

As they say, there is no replacement for displacement! So increasing engine displacement will increase the total air charge of your engine. This is in-fact the entire reason larger engines make more horsepower. Stroke and bore in this case. Turbochargers and Superchargers will also increase your air charge. Nitrous (NOX) systems also increase your air charge, although it is actually concentrated Oxygen rather than air. The impact is the same, and more oxygen allows for more fuel and combustion.

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We hope you enjoyed Part 2 of the Teaching Engine Thermodynamics Series!

Check out Teaching Engine Thermo #3 – Compression Stroke to learn about the Compression Stroke and what it means for you!

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