The Core Problem With Internal Combustion Engines

The main problem with internal combustion engines is that they burn fossil fuels, emitting large amounts of carbon dioxide and harmful air pollutants while wasting most of the fuel’s energy as heat. This combination makes them a major driver of climate change and urban air quality problems, even as engineering has steadily improved their efficiency and cleanliness.

Why Combustion Itself Is the Issue

At the heart of every internal combustion engine (ICE) is a chemical reaction: burning hydrocarbons to release energy. That reaction inevitably produces carbon dioxide (CO2), the principal long-lived greenhouse gas, along with nitrogen oxides (NOx), carbon monoxide (CO), unburned hydrocarbons, and—in the case of diesel—fine particulate matter (PM). Even the cleanest modern engines with advanced aftertreatment cannot eliminate CO2 from tailpipes because it is the direct product of combustion. Per unit fuel, that adds up quickly: roughly 2.3 kg of CO2 per liter of gasoline burned and about 2.6–2.7 kg per liter of diesel, not counting upstream emissions from extracting and refining fuel.

Inefficiency by Design

Beyond emissions, ICEs are fundamentally limited by thermodynamics and mechanical losses. In everyday driving, only a fraction of the fuel’s energy turns into motion; the rest is shed as heat and friction or lost to idling and pumping. That translates into typical on-road efficiencies around 20–30% for gasoline cars (peaks near 40% in the best designs) and 30–40% for diesels (with peaks in the low-to-mid 40s).

The points below break down where the energy commonly goes in a conventional car engine.

• Exhaust and coolant heat: The largest share, often more than half the fuel’s energy, leaves as hot gases and engine heat that radiators must dissipate.
• Pumping and throttling losses: Gasoline engines, especially at light loads, waste energy pulling air past a mostly closed throttle plate.
• Friction and accessories: Mechanical friction in pistons, bearings, and valvetrains, plus power drawn by oil pumps, water pumps, and alternators, erodes efficiency.
• Idling and low-load operation: City driving and stop-and-go traffic force engines to run where they are least efficient, burning fuel without producing useful work.
• Drivetrain losses: Transmissions, differentials, and tires consume additional energy before power reaches the road.

Taken together, these losses mean most fuel energy never reaches the wheels, locking ICE vehicles into a persistent efficiency deficit relative to electric powertrains.

Real-World Impacts

Climate: A Significant Share of Global Emissions

Transport accounts for roughly a quarter of energy-related CO2 emissions worldwide, and road vehicles are responsible for the majority of that. With ICEs still dominating the global vehicle fleet, their cumulative emissions remain a key obstacle to meeting climate targets set under national and international agreements.

Health: Pollution That Hits Close to Home

NOx and fine particles from vehicle exhaust react in the atmosphere to form ozone and secondary PM2.5, which are linked to asthma, cardiovascular disease, and premature death. Urban corridors with dense traffic typically suffer the most, putting disproportionate burdens on people living near busy roads. Modern emissions controls—catalytic converters, particulate filters, and selective catalytic reduction—have dramatically reduced per-vehicle pollution, but fleet turnover, maintenance gaps, and high-traffic exposure keep health risks significant in many cities.

Local Nuisances and Hidden Costs

ICE vehicles add noise, heat, and fuel volatility risks in dense areas. Owners face ongoing costs tied to complex mechanical systems (oil changes, exhaust treatment, and more), and countries reliant on imported crude oil remain exposed to price shocks and supply disruptions.

Why Improvements Haven’t Solved the Core Problem

Over decades, engineers have squeezed out more efficiency and cut criteria pollutants. Yet none of the incremental fixes remove the root cause: burning carbon-based fuels produces CO2. Several pathways can reduce harm, but each has limits.

The following list outlines major mitigation options and their trade-offs.

• Aftertreatment systems (catalysts, particulate filters, SCR): Highly effective on NOx, CO, and PM when functioning correctly, but they do not address CO2 and add cost and complexity.
• Hybrids and plug-in hybrids: Improve fuel economy markedly, especially in city driving, but still emit CO2 when the engine runs and depend on gasoline or diesel for part of their operation.
• Low-carbon liquid fuels (advanced biofuels): Can cut lifecycle emissions, but sustainable supply is limited and can compete with land and water for food or ecosystems.
• E-fuels (synthetic hydrocarbons made from green hydrogen and captured CO2): Potential drop-in for existing engines, yet production is energy-intensive and expensive, and scaling to mass road transport would demand vast renewable power.
• Engine advances (miller/atkinson cycles, turbocharging, waste-heat recovery): Deliver incremental gains but cannot overturn combustion’s thermodynamic constraints.

These measures can reduce impacts in the near term, especially where electrification is slow, but none fully resolves the climate problem inherent in tailpipe combustion.

Policy and Market Direction

Regulators and industry are converging on zero-emission pathways for road transport. In the United States, the Environmental Protection Agency finalized greenhouse-gas standards in 2024 for model years 2027–2032 that push automakers toward much lower fleet emissions, effectively accelerating electrification. California and several other states adopted rules targeting 100% zero-emission new light-duty vehicle sales by 2035. The European Union approved a 2035 requirement for new cars to achieve zero CO2 at the tailpipe, alongside a narrow pathway for vehicles running exclusively on certified e-fuels. The EU also adopted Euro 7 pollutant standards in 2024, adding durability and brake/tire particle limits and phasing in requirements later this decade. The United Kingdom and other markets have similar zero-emission sales trajectories. Automakers, responding to policies and consumer demand, are shifting investment toward battery-electric vehicles, while maintaining hybrids as a transitional technology.

What This Means for Consumers and Cities

For individuals and policymakers, practical choices today can meaningfully reduce the downsides of combustion while the fleet transitions toward cleaner technologies.

• Vehicle choice: Where feasible, choose battery-electric vehicles for the largest emissions cuts; consider efficient hybrids if charging access is limited.
• Driving and maintenance: Avoid unnecessary idling, keep tires inflated, and maintain emission controls to limit fuel use and pollution.
• Urban planning: Expand public transit, cycling, and walking infrastructure to cut vehicle miles traveled and improve air quality.
• Fleet and freight: Electrify buses, delivery vans, and urban trucks first, where duty cycles suit charging and health benefits are high.
• Electricity and fuels: Pair transport electrification with cleaner grids; prioritize truly sustainable biofuels and limited e-fuels for hard-to-electrify uses.

These steps can deliver immediate air-quality benefits and lower climate impacts, while aligning with long-term policy goals.

Bottom Line

The main problem with internal combustion engines is intrinsic: burning fuel creates CO2 and other pollutants, and the engines discard most of the fuel’s energy as waste heat. That dual challenge—climate impact and inefficiency—explains why policy and industry are steadily pivoting toward zero-emission powertrains.

Summary

Internal combustion engines remain widespread, but their core drawback is unavoidable: combustion produces substantial CO2 and harmful air pollutants while converting only a modest share of fuel energy into motion. Despite impressive engineering progress and cleaner fuels, these limits persist, driving global moves toward electrification and stricter emissions standards to protect climate and public health.

What is the main problem with all internal combustion engines?

Second, these engines require oil, which is mixed with the fuel to lubricate the engine’s moving parts. This oil helps create additional pollutants during the combustion process: higher emissions of hydrocarbons like benzene, which has been associated with adverse health effects, and of particulate matter (PM).

What is the most common cause of engine failure?

The most common causes of engine failure are engine overheating from a lack of proper cooling system maintenance and lack of engine oil lubrication due to low oil levels or leaks. Other common causes include ignoring warning signs like persistent engine misfires, failing sensors in modern engines, and issues with the fuel delivery system, which can all lead to severe engine damage. 
Overheating and Lack of Oil 

• Overheating: Opens in new tabA lack of sufficient coolant or a poorly maintained cooling system can cause the engine to overheat, leading to warped parts, leaking gaskets, and even seized components. 
• Low Oil Levels: Opens in new tabInsufficient oil levels lead to inadequate lubrication, causing metal parts to rub together and wear out quickly. 

Maintenance and Warning Signs

• Neglecting Routine Maintenance: Failing to get regular oil changes, replace the timing belt at recommended intervals, or service the coolant system is the most common cause of engine failure, according to one source. 
• Ignoring Warning Signs: Persistent engine misfires, issues with sensors or electronic components, and problems with the fuel system, such as clogged filters or failing pumps, should not be ignored. These can be early indicators of issues that, if unaddressed, lead to catastrophic engine damage and failure. 

Other Contributing Factors

• Faulty Ignition Coils: Opens in new tabA failed ignition coil can lead to misfires, which put added stress on other engine components. 
• Engine Electronics: Opens in new tabModern engines rely heavily on sensors and electronic control modules (ECMs) to function properly. A failing sensor can send incorrect data, leading to improper fuel mapping or other issues. 
• Fuel System Issues: Opens in new tabProblems like water in the fuel or a failing fuel pump can prevent the engine from receiving the fuel it needs, causing misfires and other problems. 

What is bad about the internal combustion engine?

Internal combustion engines create air pollution in two ways: (1) by releasing primary pollutants directly into the atmosphere and (2) by releasing direct emissions that create secondary pollution when they react chemically with elements of the atmosphere.

Why are car makers going back to combustion engines?

Instead of focusing on a purely electric future, it is looking to produce more combustion and hybrid models again. Customers in both the US and Europe have been slower to switch to electric cars than many manufacturers had hoped, due to problems with the charging infrastructure and high purchase prices.