Efficiency is a very important term thrown around when talking about modern technologies including automobiles or electronic devices and is one of the most significant concepts in physics under the study of energy and its transportation. We will be focusing on the transportation side of things and how efficiency is being debated among gasoline vehicle and electric vehicles today. Manufacturers from both sides have been developing and testing technologies to improve the efficiency of their engines and we will be discussing some of the more recent innovations along with sharing some background on what an engine (electric or thermal) really is.
What is efficiency in thermodynamics?
EFFICIENCY= (Energy output/ Energy input) *100
Efficiency, by definition, is the measure of how energy is utilized in a system or directly what amount of the energy is used for performing WORK. That ‘work’ can be anything from mechanical motion to inducing electrical current. When energy enters a system, a portion of that energy will be tasked to perform work through exerting a force over some time or distance whereas the rest of the unused energy will be wasted out through non-conservative forces like friction. An engine that is 100% efficient would have the same output energy as the input energy, where the ratio would be 1.0. However, efficiency can NEVER exceed 1.0 since that would mean creating energy, which would completely disobey the universal Law of Conservation of Energy.
In discussing heat engines we will be also be mentioning a specific type of efficiency in thermodynamics known as thermal efficiency. A heat engine is a specific form of an engine which receives high temperature heat and outputs lower temperature heat and a form of work. Talking more specifically about automobile engines, they convert the heat kinetic energy present in the input mixture (fuel + air) to motion through torque. This torque the combustion produces by pushing the piston down in the cylinder is the ‘work’ done by the whole process. In internal combustion engines, this process is a cyclic system called the Otto cycle. The Otto Cycle, named after German engineer Nikolaus Otto, includes the intake energy stroke, the gas compression stroke, the spark ignition stroke (power) and the exhaust release stroke in order. Below we have a couple of Pressure vs Volume graphical representations of a typical Otto cycle. In the study of thermodynamics and under the Ideal Gas Laws, P-V diagrams help in collecting data on total internal energy in a system(U-proportional to temperature), work done on the system or by the system (W) and heat transferred in or out the system(Q).
What is the most efficient heat engine and what’s the catch?
- The above Pressure vs Volume graphs show the temperature cycle for a typical 4-stroke combustion engine (left) and the Carnot cycle (right). Both processes consist of two sets of adiabatic strokes (no added energy from surroundings) and isothermal strokes (constant internal energy, in this case temperature). The cycle on the right is a theoretical process that is created under perfect conditions, meaning absolutely no loss of energy via friction or drag, which is practically impossible in modern engines.
- The most efficient thermal engine model has been described by the Carnot Engine following its Carnot Cycle for an ideal gas. The fundamental ideal gas law is stated by the formula PV=rNT where P=pressure (psi), V=volume (cubic inches), r= gas constant (joules/mole), N= number of moles of gas and T= gas temperature. Here the gas constant is a numerical value unique to all gases and represents the energy a gas can hold (how can it perform work on external matter) per unit mole (amount of a substance). I believe that this is an excellent concept to understand in the subject of chemistry so please follow this link if needed for more clarification (http://www.science.uwaterloo.ca/~cchieh/cact/c120/idealgas.html). Essentially what the above relationship allows engineers to do is to plot a visual relationship between Pressure, Volume and Temperature; three devices that are used very frequently when working with thermal engines using fluids to convert between energies.
- The Carnot Cycle on a Pressure vs Volume graph is a reversible process that is theoretical and could achieve the maximum possible work output for the least amount of energy added to the system. This is a completely virtual engine that cannot be created in real life due to its reversible characteristic and the conditions requiring no loss of energy to its surrounding environment after one cycle which given friction, sound and drag is impossible to conduct in a real engine. By “reversible” it is meant that the system can be returned to the exact initial states it was in before the process. A real functioning heat engine cannot be reversible since that would indicate absolutely no energy lost to the environment, as stated before. The Carnot Engine model is used simply to explain the most efficient possible theoretical heat engine, in use it will be a very slow and impractical form of engine to convert energy to different forms. An engine designed to achieve Carnot Efficiency must have no change in thermal energy disorder or Entropy in the system. Entropy is a complicated and a unique topic to learn more about, hence please follow this link:(http://hyperphysics.phy-astr.gsu.edu/hbase/Therm/entrop.html) for more in depth information.
The First Law of Thermodynamics states that the change in internal energy of a system (U) is the heat energy added to the system (Q) minus the work done by the system, or on the system if it is reversed, (W). Or simply put:
U(joules) = Q(heat added) – W(work done)
From this equation and using integral based calculus, engineers have found the efficiency of any heat engine to be derived by the following equation:
EFFICIENCY= W(work done)/Q_h = (Q_h – Q_c)/Q_h
Where Q_h = heat added to the system, Q_c = heat rejected by the system and W = Q_h – Q_c
A Carnot Engine, the most efficient theoretical engine, would have a similar structured equation however the ratio would consist of the the temperatures of the initial and final heat reservoirs rather than the heat added or rejected.
Recent Internal Combustion Advances in the Industry:
1.Mazda Skyactiv-X (SPCCI) (2019)
Mazda has been developing this unique technology for the past few years in the effort to design an efficient, low emission and practical combustion engine for the mass. The mechanics of this SPCCI based engine are the same as any diesel engine but the design will not be utilising pure heat and pressure to self combust the air after injecting fuel (in a diesel block). Here, using the same compression ratio and bore of a diesel type engine, a spark will be fired simultaneously as the air/fuel (gasoline) mixture to have increased pressure and hence release more heat after self combustion. The difference here is that the injection of fuel is not done for combustion but rather a spark will be fired before the self combustion timing of air already mixed with fuel. The advantage is that it allows for a very high, diesel like, compression ratio with gasoline type lean emissions. Mazda has been working with a type of IC engine called the HCCI (Homogeneous Charge Compression Ignition) engine which utilises pressure and temperature to combust gasoline however, with adding a spark plug for higher loads. At lower loads, the Skyactiv-X engine will be capable of 20% increase in projected fuel efficiency. In this mode, the ECU will cut out the use of spark ignition and allow for an increase in the cylinder’s compression ratio (CR). The lower CR along with a leaner a/f mixture can result in a very efficient low load scenario for engines capable of the Atkinson Cycle (which increases CR). The compression ratio of an engine is the ratio of the volume of air at peak and minimum piston displacement through one cycle of rotation. A typical average gasoline I-4 engine CR would be around 1:10 however diesels can increase it upto 1:18 or 1:16. This is the reason for why diesels are more fuel efficient than gasolines, they make more torque for one rotation of the crankshaft. If the piston travels down further in a cylinder after a/f combustion, the gases can perform more ‘work’ on the piston and , since torque is force X distance, an increase in torque output to the crankshaft can be noted.
The biggest issue with this type of design has been “knock”, which is the spontaneous combustion of the a/f mixture in the cylinder that would cause damage to the components from excess explosions between the cylinder walls. Due to high temperatures and increased pressure, the molecules can begin a combustion wave anywhere in the chamber resulting in unexpected vibrations of heat waves colliding and releasing a constant “knocking” noise. This is a phenomenon is called “pre-ignition” or “ping” and is known to be destructive to IC (internal combustion) piston engines. To prevent this, Mazda’s engine has two stages of spark ignition, where the first low voltage spark would begin a pressure wave for the a/f mixture (gasoline) and a second spark will evenly ignite the rest of the a/f molecules without the chance of random pre-ignition and causing knock. This technology being currently developed for mass production in Mazda sedans and hatchs is expected to be completed in 2019 and the Mazda 3 (pictured above as prototype vehicle) line should be the first to host this new type of engine.
2. 2019 Infinti VC Turbo (Currently under production)
The 2019 Infinti QX50 (shown above) will be the first and as of now only vehicle to host this new turbo charged variable compression (VC-Turbo) engine. Having been under development by the Nissan group for almost 20 years, the VC Turbo engine is designed to offer efficiency when needed and power when desired which has been the top priority in the automobile IC engine market today and various companies striving for the same goal. With electric infrastructure and vehicles gaining steam and practicality, the life of the gas engine is debatable and technologies like this VC Turbo by Infiniti are efforts to re-invent the gasoline engine today. The idea behind this engine is to serve dual purpose between being practical and powerful.
The designers use a multi-link system to change the compression ratio of the piston device upon command using a solenoid and actuation arm integrated to the engine build. As we discussed above, adjusting how much a piston can travel up and down in a cylinder and the a/f ratio can help in improving fuel efficiency and thermal efficiency of the engine drastically under good conditions. More work can be performed by the engine (in the form of torque) with leaner air-fuel ratio’s and higher compression ratios which increase overall efficiency.
3. 2019 Dodge RAM 1500 eTorque MGA
With electrification being a top route for automobile manufacturers in the dynamics department and the propulsion department, companies are looking to use the quick output and regenerative capabilities of electric motors to benefit the IC engine. Earlier in 2018, Dodge unveiled the new re-designed RAM 1500 with a new technology based theme in the interior and minor aerodynamic and capability updates. They also revealed new engine models for the 2019 model year which included 3.6 liter V6’s and 5.7 liter HEMI V8’s fitted with a mild hybrid system. This system simply uses an electric motor/generator in the place of a traditional engine alternator to not only generate electricity for the 12V battery but also provide additional grunt when needed. The V6’s will come with this system as standard and will produce 305hp at the wheels and 269lb-ft of torque stock and around 360 lb-ft (+90) with the electric motor. The V8’s motors would be good for an extra 130 lb-ft pf torque over the stock 396hp and 410 lb-ft. The eTorque system would help increase fuel economy under high loads and allow for more efficient stop/start situations. Another benefit is engine braking, which could use the electric motor as a generator to feed power to an auxiliary 48 volt storage battery and slow the engine’s effective RPM.