Why Catalytic converters are necessary is because fossil fuels — particularly gasoline — do not combust efficiently without combustion-enhancing technologies. For some fossil fuels, their inherent chemical composition limits combustion efficiency potential, again, particularly gasoline. 

While gasoline is the most common fuel of choice for cars, pickups, motorcycles, and small engines in the United States, gasoline is also the most finicky liquid fossil fuel. Gasoline is highly volatile and low in energy density. Furthermore, gasoline engines must use a rich fuel-to-air mixture in order to remain cool and not overheat and self-destruct.

Because gasoline engines must run rich, that means gasoline engines must waste fuel. 

If an engine is using a rich fuel mixture that means the air-to-fuel does not contain enough air to oxygenate all the hydrocarbons in a fuel. Hydrocarbons are the valuable component of fossil fuels that ignite/burn/combust. Without oxygen, hydrocarbons will not burn. Since gasoline engines require a rich fuel mixture, that means a portion of the fuel blows out the exhaust unburned. Wasting partially-burned fuel and unburned fuel is an inherent trait of gasoline engines

Gasoline engines wasting fuel — blowing it out the exhaust is intentional. 

The wasted fuel blown out the exhaust pipe of a gasoline engine is a loss of energy and energy resources. A rich fuel mixture is necessary in order for a gasoline engine to operate below catastrophic temperatures. The partially and unburned fuel inherent in rich-mixture exhaust blows out the tailpipe of a gasoline engine. The unburned hydrocarbons are toxic emissions with high global warming potential. 

As such — without the proper technologies, gasoline engines generate extreme emissions while producing a relatively low amount of mechanical energy. 

Fortunately, over the last 100 years, engineers and entrepreneurs found innovative ways to work around the limitations of gasoline and today’s gas engines run more efficiently and pollute less than ever before. Still, even though almost all gasoline engines have emissions reducing devices, gasoline engines remain highly inefficient. 

Because gasoline engines are inherently inefficient, because they must run on a rich fuel-to-air mixture catalytic converters are necessary.

The Ideal Combustion Engine vs Gasoline Engines

The ideal combustion engine, with respect to fuel efficiency and emissions, is one with two characteristics. First, an ideal combustion engine has a high compression ratio. And second, an ideal combustion engine runs on a lean fuel-to-air mixture. Both compression resistance and a lean fuel mix — a fuel mix that is highly oxygenated — equate to higher fuel efficiency and lower emissions. 

Compression ratio and a lean fuel mix are the two keys to combustion efficiency — the percentage of a fuel fed into an engine that becomes burns completely. Combustion efficiency is one of the two most critical variables with respect to reducing emissions and increasing fuel efficiency, thermal efficiency being the other. The keys to combustion efficiency are compression ratio and air-to-fuel ratio. 

Gasoline does not react to high compression ratios well nor do gasoline engines reach to a lean fuel mix well. 

As a result of the fact that gasoline has a poor compression resistance and gasoline engines cannot run on a lean air-to-fuel mixture means gasoline engines have low combustion efficiency. Because gasoline engines have low combustion efficiency, that makes designing low-emissions, fuel-efficient gasoline engines difficult to achieve.  

Why Gasoline Engines Require Low Compression Ratios and Rich Fuel Mixes  

Low compression ratios are necessary for gasoline engines because gasoline is very volatile. Gasoline requires a stable ignition environment in which to combust or it will not combust in a controlled manner. Compressing a fuel creates an unstable environment. Compressing a fuel generates heat. If the compression ratio of a gasoline-powered engine is too high, gasoline pre-combusts.  

Pre-combustion — the uncontrolled ignition of gasoline — is also known as knocking. Knocking occurs when the heat generated during the compression of gasoline causes the gasoline to combust without exposure to a flame or spark. Unlike diesel engines, gasoline engines are not compression engines. Gasoline engines are not designed to operate under compression generated combustion conditions. Gasoline engines are spark-fired meaning a spark initiates the combustion process. 

Compression combustion in a gasoline engine is a major issue. If gasoline pre-combusts in an engine, the energy generated is lost and damage can occur to the engine. Compression generated pre-combustion causes the fuel to heat quickly and to extreme temperatures. When compression combustion occurs in a gasoline engine, not only does the engine run inefficiently — as well as produce a pinging or knocking noise, — pre-combustion can damage the pistons in an engine.

The problem with the fact that gasoline has poor compression resistance is that the higher the compression ratio of an engine, the more efficiently it burns fuel and the fewer emissions the engine produces. The greater the pressure a fuel is under, the more complete the energy release when combustion occurs. So while pre-combustion actually releases more energy and reduces emissions, the fact that pre-combustion in a gasoline engine causes damage means gasoline engine engineers design motors that do not cause gasoline to combust as the result of compression. 

To prevent pre-combustion, engineers design gasoline engines with low compression ratios. Another way of looking at it is to say the compression resistance of gasoline — which is poor — determines the compression ratios of gasoline engines. It necessitates low compression ratios which waste energy and increase 

Importance of Compression Ratio for Fuel Economy and Emissions

Not only does compression ratio play a role in fuel economy, but emissions as well. 

Diesel’s capacity to withstand significant pressure — before combusting as a result of that pressure — means a much greater percentage of the hydrocarbon molecules in diesel combust than then percentage that occurs during gasoline combustion. The fact that gasoline cannot be placed under a great deal of pressure before combustion means gasoline combustion leads to partially burned hydrocarbons. 

Not only does the failure of a gasoline engine to completely burn gasoline means the engines are less fuel efficient, unburned and partially burned hydrocarbons result in many of the most toxic and dangerous emissions. 

So, the question is how to increase the pressure resistance of gasoline in order to increase combustion efficiency and thereby increase fuel efficiency and reduce emissions.

Lead Increased Pressure Resistance of Gasoline

While a low compression resistance means a fossil fuel burns incompletely, and therefore, produces greater emissions than a fuel with a high compression resistance, the biggest concern for environmentally-conscious consumers is poor gas mileage.

Early on in the manufacturing history of gasoline engines, engineers realized the importance of compression ratio. In order to increase the compression resistance of fuel, oil producers began adding lead to gasoline. Lead does not ignite nor combust, so the compression resistance of gasoline increases when supplemented with lead. That allowed car manufacturers to increase the compression ratios of the engines they produced.

Adding lead to gasoline pleased consumers because it resulted in vehicles that achieved higher fuel efficiencies. But, the dangers of lead soon became apparent, “Lead in petrol/gasoline is released during combustion into the atmosphere and eventually the soil. As it is absorbed or ingested from plants and animals that have absorbed it, lead acts as a poison. Lead interferes with the production of a variety of enzymes and metabolic processes that lower the ability to produce hemoglobin, damage kidney functioning, interfere with reproductive processes, and damage neurological development, particularly the brains of children.”

By the mid-1980s the EPA had banned the adding of lead to gasoline. By the early 1990s adding lead to gasoline was banned in almost every country in the developed world. 

However, without lead, the compression resistance of gasoline dropped back down again. And, since the compression ratios of engines dropped, so did gasoline engine fuel efficiency. Also, as the result of dropping the compression ratios of engines, the number of other emissions that leaded fuel reduced in gasoline engine emissions increased. 

Again, removing lead from gasoline actually increased vehicle emissions. 

But, gasoline’s poor compression resistance is not the only reason gasoline engine emissions are innately higher than heavier fossil fuels. In addition to gasoline’s poor compression resistance, gasoline engines cannot run on lean fuel-to-air mixtures.

Importance of Lean Fuel-to-Air Ratios and Why Gasoline Engines Must Run Rich

There is a specific ratio of air-to-fuel required to burn gasoline completely. That ratio is called the stoichiometric ratio and it is 14.7:1. For every 1 part gasoline, 14.7 parts of air are required to burn that part of gasoline completely. However, the efficiency at which an engine burns gasoline in a stoichiometric state generates so much heat that damage occurs to the engine. As a result, gasoline engines must always run a rich fuel-to-air mixture of about 12 parts air to 1 part gasoline. The difference between 

“The stress on most internal combustion engines is maximum when they are running at the stoichiometric ratio. The flame front propagates rapidly, maximizing peak pressure and temperature and generates the maximum power for a given airflow. [Gasoline] engines, when generating high power, are designed to run significantly rich of the peak temperatures and internal pressures.” 

Because gasoline engines must run rich, that means unburned fuel. Unburned and partially burned fuel constitutes some of the most toxic and dangerous combustion emissions. 

What is a Catalytic Converter?

A catalytic converter is a post-combustion emissions reduction device. Catalytic converters mount on the exhaust system downstream of the exhaust manifold. The active components, those that reduce emissions, are precious metals, a.k.a., noble metals. “Rhodium is a chemically inert noble metal that is resistant to corrosion. It is most frequently used in catalytic converters to clean auto emissions (in conjunction with its fellow PMG metals Platinum and Palladium).”

While the design of catalytic converters requires a complex engineering design, the physics of catalytic converters is relatively simple. Inside a box or cylinder mounted on the exhaust line, there are a series of flat plates stacked closely to one another. The face of each plate is perforated with honeycomb shaped holes. It is the plates inside a catalytic converter that contain precious metals. 

When a vehicle starts, exhaust begins flowing out of the engine and into the exhaust system. As the exhaust — which is extremely hot — passes through the catalytic converter, it super-heats the precious metals in the perforated plates. Once the catalysts in a catalytic converter heat to an optimal temperature, they begin burning the partially and unburned hydrocarbons that naturally occur in the exhaust. 

Simply, the heat from exhaust super-heats the noble metals in a catalytic converter and, in turn, the noble metals in a catalytic converter begin burning the unburned fuel that is in the exhaust. 

History of Catalytic Converters

Coincidentally, the commercial use of catalytic converters in automobiles began roughly the same time lead was added to fuel in order to increase its compression resistance, the early to mid-1970s. By the mid-1908s, it was becoming clear that lead was affecting the effectiveness of catalytic converters. By covering and coating the precious metals inside catalytic converters, the lead added to gasoline was making catalytic converters inert. 

The destruction of catalytic converters was just one more reason lead additives were banned from gasoline.

Today, catalytic converters are a requisite for both gasoline and diesel engines.

Difference between Catalytic Converters and Fuel Catalysts

Catalytic converters and fuel catalysts have a number of properties and characteristics in common. For one, the active components of both are precious metals. Another commonality between catalytic converters and fuel catalysts is that purpose of both — or at least in part in the case of a fuel catalyst — is to reduce emissions. 

But, while catalytic converters reduce emissions, they also reduce fuel efficiency. The back pressure created by a catalytic converter decreases the fuel thermal efficiency of an engine significantly. A fuel catalyst, on the other hand, both reduces emissions and increases fuel efficiency.

The Rentar Fuel Catalyst increases fuel efficiency by between 3% and 8% on over the road vehicles. On machines and machinery powered by a diesel engine, the increase in fuel efficiency can be as high as 15%. On boilers and furnaces, the Rentar Fuel Catalyst can increase fuel savings by 30%. 

While there is no doubt that catalytic converters are extremely important with respect to protecting the environment — and by extension, ourselves — from the harms of combustion engine emissions, saving fuel is equally important. And saving fuel is not only important for the pocketbook. The fewer fuel burned the fewer emissions produced.

Catalytic converters reduce emissions but do not reduce fuel consumption. A fuel catalyst, on the other hand, does both.