The Exchange: Brad Keselowski and John Heywood discuss the effect of cold weather on engine power.

The Arts and Sciences used to share much of the same intellectual space. Only recently have they diverged to the degree that they seem diametrically opposed. The Exchange is our attempt to rekindle some of the dialogue that occurred between the two fields.

In this installment, we’ve brought together NASCAR Cup champion and team owner Brad Keselowski and one of the world’s top internal combustion experts, MIT scientist John B. Heywood.

Bras Keselowski is an American professional stock car racing driver, team owner, and entrepreneur. He competes full-time in the NASCAR Cup Series, driving the No. 6 Ford Mustang for RFK Racing, a team he also co-owns. He was the owner of Brad Keselowski Racing, which fielded two full-time trucks in the NASCAR Camping World Truck Series.

Keselowski, who began his NASCAR career in 2004, is the second of only six drivers who have won a championship in both the Cup Series and the Xfinity Series, and the twenty-fifth driver to win a race in each of NASCAR’s three national series.

He is the owner and founder of Keselowski Advanced Manufacturing, a hybrid manufacturing company based in Statesville, North Carolina, specializing in additive metal technologies as well as CNC machining.

John B. Heywood is a British mechanical engineer known for his work on automotive engine research, for authoring a number of field-defining textbooks on the internal combustion engine, and as the director of the Sloan Automotive Lab at the Massachusetts Institute of Technology (MIT).

Heywood was elected a member of the National Academy of Engineering in 1998 for the prediction of emissions and efficiencies of spark-ignition engines and contributions to national policies on motor emissions.

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Brad Keselowski (CREDIT: Action Sports Photography)

BRAD KESELOWSKI: Why do engines make more power when its cold outside?

JOHN B. HEYWOOD: What Brad is really asking is: How does the temperature of the air (the ambient air) that enters the engine affect the engines maximum power? It’s a good question, and obviously if the effect on engine power is significant, the air’s temperature will be an important variable.

Maximum engine power occurs at high engine speed, close to the engines allowable (rated) speed limit and when the throttle (that controls the intake airflow) is wide open.  The power the engine delivers (P), is the driveshaft torque or turning moment (T) multiplied by the crankshaft (driveshaft) rotational speed (N): i.e., P = 2NT, where N is in revolutions per second. The magnitude of the torque pulses generated by each of the engine’s cylinders, each time each cylinder fires, depends on the amount of fuel chemical energy released by each cylinder’s combustion process; this release of fuel energy requires oxygen and is thus limited by the amount of air inducted into each cylinder each cycle. So the density, and thus the temperature of the air entering the cylinder, would be expected to matter.

The intake airflow is “driven” by the piston moving down the cylinder (from top to bottom) towards the crankshaft during that portion of the operating cycle when the intake valves are open. The descending piston pulls air from the atmosphere into the intake system, through the manifold and the open intake valves (usually, two of these in parallel, per cylinder, In modern engines) into each cylinder, in turn. Since the flow area of the intake manifold runners, and especially the open area of the intake valves, are many times smaller than the cross-sectional area of the descending piston, the air velocity within these intake components is much higher than the velocity of the descending piston as it pulls air into the cylinder.

Some typical numbers: For a standard-sized gasoline engine, with bore and stroke about equal at 90 mm, and with the engine at 6500 rev/min, the mean piston speed is close to 20 m/s. When the two intake valves are one-quarter open, their flow area would be about 4 cm2 (at about 2-3 mm of valve lift). Flow conservation would then require an air velocity through the open area of the valves of about 320 m/s. This is close to the speed of sound in air (speed at which pressure waves travel, 330 m/s at typical ambient temperatures). The flow is close to “choking”.

What are the consequences of this flow choking? When a gas flows through a flow restriction, as the flow area goes down the flow speeds up. As the pressure ratio across the restriction is increased, the flow velocities increase.  At the minimum area point, eventually the gas velocity reaches a maximum, the sound speed. Beyond this choking point, the flow rate can only be increases by “pushing harder” on the upstream side of the restriction (for example by using a turbocharger to raise the pressure upstream of the flow-restricting valves). Reducing the pressure in the cylinder by moving the piston faster by increasing engine speed, downstream of the open valves, will not increase the airflow because, with the intake valve flow at sonic velocity, sound waves/pressure pulses cannot propagate upstream and thus increase the flow because the flow is moving downstream at that same sonic velocity. The engine speed at which the maximum power is generated occurs when the intake airflow becomes choked over enough of the valve lift profile to significantly restrict airflow as engine speed is further increased. While engine friction which increases as engine speed increases also has some impact as to where this maximum-power speed occurs, it is the onset of airflow choking that primarily determines when the fall-off in engine power (with rising speed) occurs.

So, how does the engine intake airflow while choked, during the low-valve-lift open period, depend on air temperature? Analysis using mass and energy conservation principles shows that the choked mass-flow rate depends on the upstream pressure (no surprise), and inversely on the square root of the absolute temperature (in degrees kelvin, degrees C plus 273).

A choked flow example:  Suppose the ambient temperature drops from 20oC (68oF) to 0oC (32oF); by how much will this airflow increase? The absolute temperature decrease is from 293 to 273 kelvin. The square root of the ratio of these temperatures is 1.036. Note that in gasoline  engines, at maximum power: the intake airflow is only choked for part on the intake process; as speed increases, friction will increase; as some air heating occurs in the intake system, air temperatures will rise. So maybe about half of that 3.6% potential will be realized in the real world. 

Thus, a 20oC air temperature drop would result in maybe a 2% increase in maximum power, which might be noticeable in aggressive driving in a racing car. This, while real, would not be discernable to regular drivers in normal driving where maximum engine power is rarely used. So while the initial thought that “colder air is more dense, so more air will get into the cylinder so we can burn more fuel and generate more power” is qualitatively appropriate thinking, the details of the airflow limiting processes are much more complex and limit the practical engine power benefits in a vehicle to really modest levels. 

JOHN B. HEYWOOD: What evidence have you seen that shows that on colder days, the engines in the vehicles you race exhibits higher engine power? (E.g., do they record faster lap times?) My discussion indicates that the change in maximum engine power is likely to small enough that the effect is challenging to measure. (Note that at power levels less than the maximum, power can readily be increased by opening the throttle more, or increasing engine speed.)

BRAD KESELOWSKI: In my experience, lap times definitely decrease as the temperature drops. While the engine component is an influence here, there are several other factors that affect the power.

Beyond lap time data, there are other things we notice that point to increased power. Intakeair temperature and intake pressure both show a change in power, and typically we notice these swings in data in races that start in the day and finish at night. We also see engine operating temperature decrease as the ambient temperature drops, meaning we require less air flow to cool the engine.

Bottom line, the airflow that cools the engine is linked to the air going into the engine, so if we’re able to reduce the airflow devoted to that, we get a small increase in pressure to the engine intake.”

COVER IMAGE: Action Sports Photography.

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